Internet Calendaring and Scheduling Core Object Specification (iCalendar)

The following sections define the details of a Calendaring and Scheduling Core Object Specification. The Calendaring and Scheduling Core Object is a collection of calendaring and scheduling information. Typically, this information will consist of an iCalendar stream with one or more iCalendar objects. The body of the iCalendar object consists of a sequence of calendar properties and one or more calendar components. defines the content line format; defines the property parameter format; defines the data types for property values; defines the iCalendar object format; defines the iCalendar property format; defines the calendar component format; defines calendar properties; and defines calendar component properties. This information is intended to be an integral part of the MIME content type registration. In addition, this information can be used independent of such content registration. In particular, this memo has direct applicability for use as a calendaring and scheduling exchange format in file-, memory- or network-based transport mechanisms.

The iCalendar object is organized into individual lines of text, called content lines. Content lines are delimited by a line break, which is a CRLF sequence (US-ASCII decimal 13, followed by US-ASCII decimal 10). Lines of text SHOULD NOT be longer than 75 octets, excluding the line break. Long content lines SHOULD be split into a multiple line representations using a line "folding" technique. That is, a long line can be split between any two characters by inserting a CRLF immediately followed by a single linear white space character (i.e., SPACE, US-ASCII decimal 32 or HTAB, US-ASCII decimal 9). Any sequence of CRLF followed immediately by a single linear white space character is ignored (i.e., removed) when processing the content type. For example the line:

Can be represented as:

The process of moving from this folded multiple line representation to its single line representation is called "unfolding". Unfolding is accomplished by removing the CRLF character and the linear white space character that immediately follows. When parsing a content line, folded lines MUST first be unfolded according to the unfolding procedure described above. Note: It is possible for very simple implementations to generate improperly folded lines in the middle of a UTF-8 multi-octet sequence. For this reason, implementations need to unfold lines in such a way to properly restore the original sequence. The content information associated with an iCalendar object is formatted using a syntax similar to that defined by . That is, the content information consists of CRLF-separated content lines. The following notation defines the lines of content in an iCalendar object:

The property value component of a content line has a format that is property specific. Refer to the section describing each property for a definition of this format. All names of properties, property parameters, enumerated property values and property parameter values are case-insensitive. However, all other property values are case-sensitive, unless otherwise stated.

Some properties and parameters allow a list of values. Values in a list of values MUST be separated by a COMMA character (US-ASCII decimal 44). There is no significance to the order of values in a list. For those parameter values (such as those that specify URI values) that are specified in quoted-strings, the individual quoted- strings are separated by a COMMA character (US-ASCII decimal 44). Some property values are defined in terms of multiple parts. These structured property values MUST have their value parts separated by a SEMICOLON character (US-ASCII decimal 59). Some properties allow a list of parameters. Each property parameter in a list of property parameters MUST be separated by a SEMICOLON character (US-ASCII decimal 59). Property parameters with values containing a COLON character (US-ASCII decimal 58), a SEMICOLON character (US-ASCII decimal 59) or a COMMA character (US-ASCII decimal 44) MUST be placed in quoted text. For example, in the following properties a SEMICOLON is used to separate property parameters from each other, and a COMMA is used to separate property values in a value list.

Some properties defined in the iCalendar object can have multiple values. The general rule for encoding multi-valued items is to simply create a new content line for each value, including the property name. However, it should be noted that some properties support encoding multiple values in a single property by separating the values with a COMMA character (US-ASCII decimal 44). Individual property definitions should be consulted for determining whether a specific property allows multiple values and in which of these two forms. Multi-valued properties MUST NOT be used to specify multiple language variants of the same value. Calendar applications SHOULD display all values.

Binary content information in an iCalendar object SHOULD be referenced using a URI within a property value. That is the binary content information SHOULD be placed in an external MIME entity that can be referenced by a URI from within the iCalendar object. In applications where this is not feasible, binary content information can be included within an iCalendar object, but only after first encoding it into text using the "BASE64" encoding method defined in . Inline binary content SHOULD only be used in applications whose special circumstances demand that an iCalendar object be expressed as a single entity. A property containing inline binary content information MUST specify the "ENCODING" property parameter. Binary content information placed external to the iCalendar object MUST be referenced by a uniform resource identifier (URI). The following example specifies an "ATTACH" property that references an attachment external to the iCalendar object with a URI reference:

The following example specifies an "ATTACH" property with inline binary encoded content information:

There is not a property parameter to declare the charset used in a property value. The default charset for an iCalendar stream is UTF-8 as defined in . The "charset" Content-Type parameter MUST be used in MIME transports to specify the charset being used.

A property can have attributes associated with it. These "property parameters" contain meta-information about the property or the property value. Property parameters are provided to specify such information as the location of an alternate text representation for a property value, the language of a text property value, the value type of the property value and other attributes. Property parameter values that contain the COLON (US-ASCII decimal 58), SEMICOLON (US-ASCII decimal 59) or COMMA (US-ASCII decimal 44) character separators MUST be specified as quoted-string text values. Property parameter values MUST NOT contain the DQUOTE (US-ASCII decimal 22) character. The DQUOTE (US-ASCII decimal 22) character is used as a delimiter for parameter values that contain restricted characters or URI text. For example:

Property parameter values that are not in quoted strings are case insensitive. The general property parameters defined by this memo are defined by the following notation:

Applications MUST ignore x-param and iana-param value they don't recognized.

ALTREP To specify an alternate text representation for the property value. This property parameter is defined by the following notation:

This parameter specifies a URI that points to an alternate representation for a textual property value. A property specifying this parameter MUST also include a value that reflects the default representation of the text value. The URI parameter value MUST be specified in a quoted-string. Note: While there is no restriction imposed on the URI schemes allowed for this parameter, CID, HTTP, and HTTPS are the URI schemes most commonly used by current implementations.

The "ALTREP" property parameter value might point to a "text/html" content portion.

Project XYZ Review Meeting will include the following agenda items:

Market Overview
Finances
Project Management

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CN To specify the common name to be associated with the calendar user specified by the property. This property parameter is defined by the following notation:

This parameter can be specified on properties with a CAL-ADDRESS value type. The parameter specifies the common name to be associated with the calendar user specified by the property. The parameter value is text. The parameter value can be used for display text to be associated with the calendar address specified by the property.

CUTYPE To specify the type of calendar user specified by the property. This property parameter is defined by the following notation:

This parameter can be specified on properties with a CAL-ADDRESS value type. The parameter identifies the type of calendar user specified by the property. If not specified on a property that allows this parameter, the default is INDIVIDUAL. Applications MUST treat x-name and iana-token value they don't recognized the same way as they would the UNKNOWN value.

DELEGATED-FROM To specify the calendar users that have delegated their participation to the calendar user specified by the property. This property parameter is defined by the following notation:

This parameter can be specified on properties with a CAL-ADDRESS value type. This parameter specifies those calendar users that have delegated their participation in a group scheduled event or to-do to the calendar user specified by the property. The individual calendar address parameter values MUST each be specified in a quoted-string.

DELEGATED-TO To specify the calendar users to whom the calendar user specified by the property has delegated participation. This property parameter is defined by the following notation:

This parameter can be specified on properties with a CAL-ADDRESS value type. This parameter specifies those calendar users whom have been delegated participation in a group scheduled event or to-do by the calendar user specified by the property. The individual calendar address parameter values MUST each be specified in a quoted-string.

DIR To specify reference to a directory entry associated with the calendar user specified by the property. This property parameter is defined by the following notation:

This parameter can be specified on properties with a CAL-ADDRESS value type. The parameter specifies a reference to the directory entry associated with the calendar user specified by the property. The parameter value is a URI. The URI parameter value MUST be specified in a quoted-string. Note: While there is no restriction imposed on the URI schemes allowed for this parameter, CID, DATA, FILE, FTP, HTTP, HTTPS, LDAP, and MID are the URI schemes most commonly used by current implementations.

ENCODING To specify an alternate inline encoding for the property value. This property parameter is defined by the following notation:

This property parameter identifies the inline encoding used in a property value. The default encoding is "8BIT", corresponding to a property value consisting of text. The "BASE64" encoding type corresponds to a property value encoded using the "BASE64" encoding defined in . If the value type parameter is ";VALUE=BINARY", then the inline encoding parameter MUST be specified with the value ";ENCODING=BASE64".

FMTTYPE To specify the content type of a referenced object. This property parameter is defined by the following notation:

This parameter can be specified on properties that are used to reference an object. The parameter specifies the media type of the referenced object. For example, on the "ATTACH" property, a FTP type URI value does not, by itself, necessarily convey the type of content associated with the resource. The parameter value MUST be the text for either an IANA registered media type or a non-standard media type.

FBTYPE To specify the free or busy time type. This property parameter is defined by the following notation:

This parameter specifies the free or busy time type. The value FREE indicates that the time interval is free for scheduling. The value BUSY indicates that the time interval is busy because one or more events have been scheduled for that interval. The value BUSY-UNAVAILABLE indicates that the time interval is busy and that the interval can not be scheduled. The value BUSY-TENTATIVE indicates that the time interval is busy because one or more events have been tentatively scheduled for that interval. If not specified on a property that allows this parameter, the default is BUSY. Applications MUST treat x-name and iana-token value they don't recognized the same way as they would the BUSY value. The following is an example of this parameter on a "FREEBUSY" property.

LANGUAGE To specify the language for text values in a property or property parameter. This property parameter is defined by the following notation:

This parameter identifies the language of the text in the property value and of all property parameter values of the property. The value of the "LANGUAGE" property parameter is that defined in . For transport in a MIME entity, the Content-Language header field can be used to set the default language for the entire body part. Otherwise, no default language is assumed. The following are examples of this parameter on the "SUMMARY" and "LOCATION" properties:

MEMBER To specify the group or list membership of the calendar user specified by the property. This property parameter is defined by the following notation:

This parameter can be specified on properties with a CAL-ADDRESS value type. The parameter identifies the groups or list membership for the calendar user specified by the property. The parameter value is either a single calendar address in a quoted-string or a COMMA character (US-ASCII decimal 44) separated list of calendar addresses, each in a quoted-string. The individual calendar address parameter values MUST each be specified in a quoted-string.

PARTSTAT To specify the participation status for the calendar user specified by the property. This property parameter is defined by the following notation:

This parameter can be specified on properties with a CAL-ADDRESS value type. The parameter identifies the participation status for the calendar user specified by the property value. The parameter values differ depending on whether they are associated with a group scheduled "VEVENT", "VTODO" or "VJOURNAL". The values MUST match one of the values allowed for the given calendar component. If not specified on a property that allows this parameter, the default value is NEEDS-ACTION. Applications MUST treat x-name and iana-token value they don't recognized the same way as they would the NEEDS-ACTION value.

RANGE To specify the effective range of recurrence instances from the instance specified by the recurrence identifier specified by the property. This property parameter is defined by the following notation:

This parameter can be specified on a property that specifies a recurrence identifier. The parameter specifies the effective range of recurrence instances that is specified by the property. The effective range is from the recurrence identifier specified by the property. If this parameter is not specified on an allowed property, then the default range is the single instance specified by the recurrence identifier value of the property. The parameter value can only be "THISANDFUTURE" to indicate a range defined by the recurrence identifier and all subsequent instances. The value "THISANDPRIOR" is deprecated by this revision of iCalendar and MUST NOT be generated by applications.

RELATED To specify the relationship of the alarm trigger with respect to the start or end of the calendar component. This property parameter is defined by the following notation:

This parameter can be specified on properties that specify an alarm trigger with a "DURATION" value type. The parameter specifies whether the alarm will trigger relative to the start or end of the calendar component. The parameter value START will set the alarm to trigger off the start of the calendar component; the parameter value END will set the alarm to trigger off the end of the calendar component. If the parameter is not specified on an allowable property, then the default is START.

RELTYPE To specify the type of hierarchical relationship associated with the calendar component specified by the property. This property parameter is defined by the following notation:

This parameter can be specified on a property that references another related calendar. The parameter specifies the hierarchical relationship type of the calendar component referenced by the property. The parameter value can be PARENT, to indicate that the referenced calendar component is a superior of calendar component; CHILD to indicate that the referenced calendar component is a subordinate of the calendar component; SIBLING to indicate that the referenced calendar component is a peer of the calendar component. If this parameter is not specified on an allowable property, the default relationship type is PARENT. Applications MUST treat x-name and iana-token value they don't recognized the same way as they would the PARENT value.

ROLE To specify the participation role for the calendar user specified by the property. This property parameter is defined by the following notation:

This parameter can be specified on properties with a CAL-ADDRESS value type. The parameter specifies the participation role for the calendar user specified by the property in the group schedule calendar component. If not specified on a property that allows this parameter, the default value is REQ-PARTICIPANT. Applications MUST treat x-name and iana-token value they don't recognized the same way as they would the REQ-PARTICIPANT value.

RSVP To specify whether there is an expectation of a favor of a reply from the calendar user specified by the property value. This property parameter is defined by the following notation:

This parameter can be specified on properties with a CAL-ADDRESS value type. The parameter identifies the expectation of a reply from the calendar user specified by the property value. This parameter is used by the "Organizer" to request a participation status reply from an "Attendee" of a group scheduled event or to-do. If not specified on a property that allows this parameter, the default value is FALSE.

SENT-BY To specify the calendar user that is acting on behalf of the calendar user specified by the property. This property parameter is defined by the following notation:

This parameter can be specified on properties with a CAL-ADDRESS value type. The parameter specifies the calendar user that is acting on behalf of the calendar user specified by the property. The parameter value MUST be a mailto URI as defined in . The individual calendar address parameter values MUST each be specified in a quoted-string.

TZID To specify the identifier for the time zone definition for a time component in the property value. This property parameter is defined by the following notation:

This parameter MUST be specified on the "DTSTART", "DTEND", "DUE", "EXDATE" and "RDATE" properties when either a DATE-TIME or TIME value type is specified and when the value is not either a UTC or a "floating" time. Refer to the DATE-TIME or TIME value type definition for a description of UTC and "floating time" formats. This property parameter specifies a text value which uniquely identifies the "VTIMEZONE" calendar component to be used when evaluating the time portion of the property. The value of the "TZID" property parameter will be equal to the value of the "TZID" property for the matching time zone definition. An individual "VTIMEZONE" calendar component MUST be specified for each unique "TZID" parameter value specified in the iCalendar object. The parameter MUST be specified on properties with a DATE-TIME value if the DATE-TIME is not either a UTC or a "floating" time. The presence of the SOLIDUS character (US-ASCII decimal 47) as a prefix, indicates that this "TZID" represents a unique ID in a globally defined time zone registry (when such registry is defined). Note: This document does not define a naming convention for time zone identifiers. Implementers may want to use the naming conventions defined in existing time zone specifications such as the public-domain TZ database . The specification of globally unique time zone identifiers is not addressed by this document and is left for future study. The following are examples of this property parameter:

The "TZID" property parameter MUST NOT be applied to DATE properties, and DATE-TIME or TIME properties whose time values are specified in UTC. The use of local time in a DATE-TIME or TIME value without the "TZID" property parameter is to be interpreted as floating time, regardless of the existence of "VTIMEZONE" calendar components in the iCalendar object. For more information see the sections on the value types DATE-TIME and TIME.

VALUE To explicitly specify the value type format for a property value. This property parameter is defined by the following notation:

This parameter specifies the value type and format of the property value. The property values MUST be of a single value type. For example, a "RDATE" property cannot have a combination of DATE-TIME and TIME value types. If the property's value is the default value type, then this parameter need not be specified. However, if the property's default value type is overridden by some other allowable value type, then this parameter MUST be specified. Applications MUST preserve the value data for x-name and iana-token values that they don't recognize without attempting to interpret or parse the value data.

The properties in an iCalendar object are strongly typed. The definition of each property restricts the value to be one of the value data types, or simply value types, defined in this section. The value type for a property will either be specified implicitly as the default value type or will be explicitly specified with the "VALUE" parameter. If the value type of a property is one of the alternate valid types, then it MUST be explicitly specified with the "VALUE" parameter.

BINARY This value type is used to identify properties that contain a character encoding of inline binary data. For example, an inline attachment of a document might be included in an iCalendar object. This value type is defined by the following notation:

Property values with this value type MUST also include the inline encoding parameter sequence of ";ENCODING=BASE64". That is, all inline binary data MUST first be character encoded using the "BASE64" encoding method defined in . No additional content value encoding (i.e., BACKSLASH character encoding, see ) is defined for this value type. The following is an example of a "BASE64" encoded binary value data.

BOOLEAN This value type is used to identify properties that contain either a "TRUE" or "FALSE" Boolean value. This value type is defined by the following notation:

These values are case insensitive text. No additional content value encoding (i.e., BACKSLASH character encoding, see ) is defined for this value type. The following is an example of a hypothetical property that has a BOOLEAN value type:

CAL-ADDRESS This value type is used to identify properties that contain a calendar user address. This value type is defined by the following notation:

The value is a URI as defined by or any other IANA registered form for a URI. When used to address an Internet email transport address for a calendar user, the value MUST be a mailto URI, as defined by . No additional content value encoding (i.e., BACKSLASH character encoding, see ) is defined for this value type.

DATE This value type is used to identify values that contain a calendar date. This value type is defined by the following notation:

If the property permits, multiple "date" values are specified as a COMMA character (US-ASCII decimal 44) separated list of values. The format for the value type is based on the complete representation, basic format for a calendar date. The textual format specifies a four-digit year, two-digit month, and two-digit day of the month. There are no separator characters between the year, month and day component text. No additional content value encoding (i.e., BACKSLASH character encoding, see ) is defined for this value type. The following represents July 14, 1997:

DATE-TIME This value type is used to identify values that specify a precise calendar date and time of day. This value type is defined by the following notation:

If the property permits, multiple "date-time" values are specified as a COMMA character (US-ASCII decimal 44) separated list of values. No additional content value encoding (i.e., BACKSLASH character encoding, see ) is defined for this value type. The "DATE-TIME" value type is used to identify values that contain a precise calendar date and time of day. The format is based on the complete representation, basic format for a calendar date and time of day. The text format is a concatenation of the "date", followed by the LATIN CAPITAL LETTER T character (US-ASCII decimal 84) time designator, followed by the "time" format. The "DATE-TIME" value type expresses time values in three forms: The form of date and time with UTC offset MUST NOT be used. For example, the following is not valid for a date-time value:

FORM #1: DATE WITH LOCAL TIME The date with local time form is simply a DATE-TIME value that does not contain the UTC designator nor does it reference a time zone. For example, the following represents January 18, 1998, at 11 PM:

DATE-TIME values of this type are said to be "floating" and are not bound to any time zone in particular. They are used to represent the same hour, minute, and second value regardless of which time zone is currently being observed. For example, an event can be defined that indicates that an individual will be busy from 11:00 AM to 1:00 PM every day, no matter which time zone the person is in. In these cases, a local time can be specified. The recipient of an iCalendar object with a property value consisting of a local time, without any relative time zone information, SHOULD interpret the value as being fixed to whatever time zone the "ATTENDEE" is in at any given moment. This means that two "Attendees", in different time zones, receiving the same event definition as a floating time, may be participating in the event at different actual times. Floating time SHOULD only be used where that is the reasonable behavior. In most cases, a fixed time is desired. To properly communicate a fixed time in a property value, either UTC time or local time with time zone reference MUST be specified. The use of local time in a DATE-TIME value without the "TZID" property parameter is to be interpreted as floating time, regardless of the existence of "VTIMEZONE" calendar components in the iCalendar object. FORM #2: DATE WITH UTC TIME The date with UTC time, or absolute time, is identified by a LATIN CAPITAL LETTER Z suffix character (US-ASCII decimal 90), the UTC designator, appended to the time value. For example, the following represents January 19, 1998, at 0700 UTC:

The "TZID" property parameter MUST NOT be applied to DATE-TIME properties whose time values are specified in UTC. FORM #3: DATE WITH LOCAL TIME AND TIME ZONE REFERENCE The date and local time with reference to time zone information is identified by the use the "TZID" property parameter to reference the appropriate time zone definition. "TZID" is discussed in detail in . For example, the following represents 2:00 A.M. in New York on Janurary 19, 1998:

If, based on the definition of the referenced time zone, the local time described occurs more than once (when changing from daylight to standard time), the DATE-TIME value refers to the first occurence of the referenced time. Thus, TZID=America/New_York:20071104T013000 indicates November 4, 2007 at 1:30 A.M. EDT (UTC-04:00). If the local time described does not occur (when changing from standard to daylight time), the DATE-TIME value is interpreted using the UTC offset before the gap in local times. Thus, TZID=America/New_York:20070311T023000 indicates March 11, 2007 at 3:30 A.M. EDT (UTC-04:00), one hour after 1:30 A.M. EST (UTC-05:00). A time value MUST only specify the second 60 when specifying a positive leap second. For example:

Implementations which do not support leap seconds SHOULD interpret the second 60 as equivalent to the second 59. The following represents July 14, 1997, at 1:30 PM in New York City in each of the three time formats, using the "DTSTART" property.

DURATION This value type is used to identify properties that contain a duration of time. This value type is defined by the following notation:

If the property permits, multiple "duration" values are specified by a COMMA character (US-ASCII decimal 44) separated list of values. The format is based on the complete representation basic format with designators for the duration of time. The format can represent nominal durations (weeks and days) and accurate durations (hours, minutes, and seconds). Note that unlike this value type doesn't support the "Y" and "M" designators to specify durations in terms of years and months. The duration of a week or a day depends on its position in the calendar. In the case of discontinuities in the time scale, such as the change from standard time to daylight time and back, the computation of the exact duration requires the substraction or addition of the change of duration of the discontinuity. Leap seconds MUST NOT be considered when computing an exact duration. When computing an exact duration, the greatest order time components MUST be added first, that is, the number of days MUST be added first, followed by the number of hours, number of minutes, and number of seconds. Negative durations are typically used to schedule an alarm to trigger before an associated time (see ). No additional content value encoding (i.e., BACKSLASH character encoding, see ) are defined for this value type. A duration of 15 days, 5 hours and 20 seconds would be:

A duration of 7 weeks would be:

FLOAT This value type is used to identify properties that contain a real number value. This value type is defined by the following notation:

If the property permits, multiple "float" values are specified by a COMMA character (US-ASCII decimal 44) separated list of values. No additional content value encoding (i.e., BACKSLASH character encoding, see ) is defined for this value type.

INTEGER This value type is used to identify properties that contain a signed integer value. This value type is defined by the following notation:

If the property permits, multiple "integer" values are specified by a COMMA character (US-ASCII decimal 44) separated list of values. The valid range for "integer" is -2147483648 to 2147483647. If the sign is not specified, then the value is assumed to be positive. No additional content value encoding (i.e., BACKSLASH character encoding, see ) is defined for this value type.

PERIOD This value type is used to identify values that contain a precise period of time. This value type is defined by the following notation:

If the property permits, multiple "period" values are specified by a COMMA character (US-ASCII decimal 44) separated list of values. There are two forms of a period of time. First, a period of time is identified by its start and its end. This format is based on the complete representation, basic format for "DATE-TIME" start of the period, followed by a SOLIDUS character (US-ASCII decimal 47), followed by the "DATE-TIME" of the end of the period. The start of the period MUST be before the end of the period. Second, a period of time can also be defined by a start and a positive duration of time. The format is based on the complete representation, basic format for the "DATE-TIME" start of the period, followed by a SOLIDUS character (US-ASCII decimal 47), followed by the basic format for "DURATION" of the period. The period starting at 18:00:00 UTC, on January 1, 1997 and ending at 07:00:00 UTC on January 2, 1997 would be:

The period start at 18:00:00 on January 1, 1997 and lasting 5 hours and 30 minutes would be:

No additional content value encoding (i.e., BACKSLASH character encoding, see ) is defined for this value type.

RECUR This value type is used to identify properties that contain a recurrence rule specification. This value type is defined by the following notation:

This value type is a structured value consisting of a list of one or more recurrence grammar parts. Each rule part is defined by a NAME=VALUE pair. The rule parts are separated from each other by the SEMICOLON character (US-ASCII decimal 59). The rule parts are not ordered in any particular sequence. Individual rule parts MUST only be specified once. Compliant applications MUST accept rule parts ordered in any sequence, but to ensure backward compatibility with applications that pre-date this revision of iCalendar the FREQ rule part MUST be the first rule part specified in a RECUR value. The FREQ rule part identifies the type of recurrence rule. This rule part MUST be specified in the recurrence rule. Valid values include SECONDLY, to specify repeating events based on an interval of a second or more; MINUTELY, to specify repeating events based on an interval of a minute or more; HOURLY, to specify repeating events based on an interval of an hour or more; DAILY, to specify repeating events based on an interval of a day or more; WEEKLY, to specify repeating events based on an interval of a week or more; MONTHLY, to specify repeating events based on an interval of a month or more; and YEARLY, to specify repeating events based on an interval of a year or more. The INTERVAL rule part contains a positive integer representing at which intervals the recurrence rule repeats. The default value is "1", meaning every second for a SECONDLY rule, every minute for a MINUTELY rule, every hour for an HOURLY rule, every day for a DAILY rule, every week for a WEEKLY rule, every month for a MONTHLY rule, and every year for a YEARLY rule. For example, within a DAILY rule, a value of "8" means every eight days. The UNTIL rule part defines a DATE or DATE-TIME value which bounds the recurrence rule in an inclusive manner. If the value specified by UNTIL is synchronized with the specified recurrence, this DATE or DATE-TIME becomes the last instance of the recurrence. The value of the UNTIL rule part MUST have the same value type as the "DTSTART" property. Furthermore, if the "DTSTART" property is specified as a date with local time, then the UNTIL rule part MUST also be specified as a date with local time. If the "DTSTART" property is specified as a date with UTC time or a date with local time and time zone reference, then the UNTIL rule part MUST be specified as a date with UTC time. In the case of the "STANDARD" and "DAYLIGHT" sub-components the UNTIL rule part MUST always be specified as a date with UTC time. If specified as a DATE-TIME value, then it MUST be specified in a UTC time format. If not present, and the COUNT rule part is also not present, the "RRULE" is considered to repeat forever. The COUNT rule part defines the number of occurrences at which to range-bound the recurrence. The "DTSTART" property value always counts as the first occurrence. The BYSECOND rule part specifies a COMMA character (US-ASCII decimal 44) separated list of seconds within a minute. Valid values are 0 to 60. The BYMINUTE rule part specifies a COMMA character (US-ASCII decimal 44) separated list of minutes within an hour. Valid values are 0 to 59. The BYHOUR rule part specifies a COMMA character (US-ASCII decimal 44) separated list of hours of the day. Valid values are 0 to 23. The BYSECOND, BYMINUTE and BYHOUR rule parts MUST NOT be specified when the associated "DTSTART" property has a DATE value type. These rule parts MUST be ignored in RECUR value that violate the above requirement (e.g., generated by applications that pre-date this revision of iCalendar). The BYDAY rule part specifies a COMMA character (US-ASCII decimal 44) separated list of days of the week; SU indicates Sunday; MO indicates Monday; TU indicates Tuesday; WE indicates Wednesday; TH indicates Thursday; FR indicates Friday; SA indicates Saturday. Each BYDAY value can also be preceded by a positive (+n) or negative (-n) integer. If present, this indicates the nth occurrence of a specific day within the MONTHLY or YEARLY "RRULE". For example, within a MONTHLY rule, +1MO (or simply 1MO) represents the first Monday within the month, whereas -1MO represents the last Monday of the month. The numeric value in a BYDAY rule part with the FREQ rule part set to YEARLY corresponds to an offset within the month when the BYMONTH rule part is present, and corresponds to an offset within the year when the BYWEEKNO or BYMONTH rule parts are present. If an integer modifier is not present, it means all days of this type within the specified frequency. For example, within a MONTHLY rule, MO represents all Mondays within the month. The BYDAY rule part MUST NOT be specified with a numeric value when the FREQ rule part is not set to MONTHLY or YEARLY. Furthermore, the BYDAY rule part MUST NOT be specified with a numeric value with the FREQ rule part set to YEARLY when the BYWEEKNO rule part is specified. The BYMONTHDAY rule part specifies a COMMA character (US-ASCII decimal 44) separated list of days of the month. Valid values are 1 to 31 or -31 to -1. For example, -10 represents the tenth to the last day of the month. The BYMONTHDAY rule part MUST NOT be specified when the FREQ rule part is set to WEEKLY. The BYYEARDAY rule part specifies a COMMA character (US-ASCII decimal 44) separated list of days of the year. Valid values are 1 to 366 or -366 to -1. For example, -1 represents the last day of the year (December 31st) and -306 represents the 306th to the last day of the year (March 1st). The BYYEARDAY rule part MUST NOT be specified when the FREQ rule part is set to DAILY, WEEKLY, or MONTHLY. The BYWEEKNO rule part specifies a COMMA character (US-ASCII decimal 44) separated list of ordinals specifying weeks of the year. Valid values are 1 to 53 or -53 to -1. This corresponds to weeks according to week numbering as defined in . A week is defined as a seven day period, starting on the day of the week defined to be the week start (see WKST). Week number one of the calendar year is the first week which contains at least four (4) days in that calendar year. This rule part MUST NOT be used when the FREQ rule part is set to anything other than YEARLY. For example, 3 represents the third week of the year. Note: Assuming a Monday week start, week 53 can only occur when Thursday is January 1 or if it is a leap year and Wednesday is January 1. The BYMONTH rule part specifies a COMMA character (US-ASCII decimal 44) separated list of months of the year. Valid values are 1 to 12. The WKST rule part specifies the day on which the workweek starts. Valid values are MO, TU, WE, TH, FR, SA and SU. This is significant when a WEEKLY "RRULE" has an interval greater than 1, and a BYDAY rule part is specified. This is also significant when in a YEARLY "RRULE" when a BYWEEKNO rule part is specified. The default value is MO. The BYSETPOS rule part specifies a COMMA character (US-ASCII decimal 44) separated list of values which corresponds to the nth occurrence within the set of recurrence instances specified by the rule. BYSETPOS operates on a set of recurrence instances in one interval of the recurrence rule. For example, in a WEEKLY rule, the interval would be one week A set of recurrence instances starts at the beginning of the interval defined by the FREQ rule part. Valid values are 1 to 366 or -366 to -1. It MUST only be used in conjunction with another BYxxx rule part. For example "the last work day of the month" could be represented as:

Each BYSETPOS value can include a positive (+n) or negative (-n) integer. If present, this indicates the nth occurrence of the specific occurrence within the set of occurences specified by the rule. Recurrence rules may generate recurrence instances with invalid date (e.g., February 30) or nonexistent local time (e.g., 1:30 AM on a day where the local time is moved forward by an hour at 1:00 AM). Such recurrence instances MUST be ignored and MUST NOT be counted as part of the recurrence set. Information, not contained in the rule, necessary to determine the various recurrence instance start time and dates are derived from the Start Time ("DTSTART") component attribute. For example, "FREQ=YEARLY;BYMONTH=1" doesn't specify a specific day within the month or a time. This information would be the same as what is specified for "DTSTART". BYxxx rule parts modify the recurrence in some manner. BYxxx rule parts for a period of time which is the same or greater than the frequency generally reduce or limit the number of occurrences of the recurrence generated. For example, "FREQ=DAILY;BYMONTH=1" reduces the number of recurrence instances from all days (if BYMONTH rule part is not present) to all days in January. BYxxx rule parts for a period of time less than the frequency generally increase or expand the number of occurrences of the recurrence. For example, "FREQ=YEARLY;BYMONTH=1,2" increases the number of days within the yearly recurrence set from 1 (if BYMONTH rule part is not present) to 2. If multiple BYxxx rule parts are specified, then after evaluating the specified FREQ and INTERVAL rule parts, the BYxxx rule parts are applied to the current set of evaluated occurrences in the following order: BYMONTH, BYWEEKNO, BYYEARDAY, BYMONTHDAY, BYDAY, BYHOUR, BYMINUTE, BYSECOND and BYSETPOS; then COUNT and UNTIL are evaluated. The table below summarizes the dependency of BYxxx rule part expand or limit behaviour on the FREQ rule part value. The term "N/A" means that the corresponding BYxxx rule part MUST NOT be used with the corresponding FREQ value. BYDAY has some special behaviour depending on the FREQ value and this is described in separate notes below the table.

Limit if BYMONTHDAY is present, otherwise special expand for MONTHLY. Limit if BYYEARDAY or BYMONTHDAY is present, otherwise special expand for WEEKLY if BYWEEKNO present, otherwise special expand for MONTHLY if BYMONTH present, otherwise special expand for YEARLY. Here is an example of evaluating multiple BYxxx rule parts.

First, the "INTERVAL=2" would be applied to "FREQ=YEARLY" to arrive at "every other year". Then, "BYMONTH=1" would be applied to arrive at "every January, every other year". Then, "BYDAY=SU" would be applied to arrive at "every Sunday in January, every other year". Then, "BYHOUR=8,9" would be applied to arrive at "every Sunday in January at 8 AM and 9 AM, every other year". Then, "BYMINUTE=30" would be applied to arrive at "every Sunday in January at 8:30 AM and 9:30 AM, every other year". Then, lacking information from "RRULE", the second is derived from "DTSTART", to end up in "every Sunday in January at 8:30:00 AM and 9:30:00 AM, every other year". Similarly, if the BYMINUTE, BYHOUR, BYDAY, BYMONTHDAY or BYMONTH rule part were missing, the appropriate minute, hour, day or month would have been retrieved from the "DTSTART" property. If the computed local start time of a recurrence instance does not exist, or occurs more than once, for the specified time zone, the time of the recurrence instance is interpreted in the same manner as an explicit DATE-TIME value describing that date and time, as specified in . No additional content value encoding (i.e., BACKSLASH character encoding, see ) is defined for this value type. The following is a rule which specifies 10 occurences which occur every other day:

There are other examples specified in .

TEXT This value type is used to identify values that contain human readable text. This value type is defined by the following notation.

If the property permits, multiple TEXT values are specified by a COMMA character (US-ASCII decimal 44) separated list of values. The language in which the text is represented can be controlled by the "LANGUAGE" property parameter. An intentional formatted text line break MUST only be included in a "TEXT" property value by representing the line break with the character sequence of BACKSLASH (US-ASCII decimal 92), followed by a LATIN SMALL LETTER N (US-ASCII decimal 110) or a LATIN CAPITAL LETTER N (US-ASCII decimal 78), that is "\n" or "\N". The "TEXT" property values may also contain special characters that are used to signify delimiters, such as a COMMA character for lists of values or a SEMICOLON character for structured values. In order to support the inclusion of these special characters in "TEXT" property values, they MUST be escaped with a BACKSLASH character. A BACKSLASH character (US-ASCII decimal 92) in a "TEXT" property value MUST be escaped with another BACKSLASH character. A COMMA character in a "TEXT" property value MUST be escaped with a BACKSLASH character (US-ASCII decimal 92). A SEMICOLON character in a "TEXT" property value MUST be escaped with a BACKSLASH character (US-ASCII decimal 92). However, a COLON character in a "TEXT" property value SHALL NOT be escaped with a BACKSLASH character. A multiple line value of:

would be represented as:

TIME This value type is used to identify values that contain a time of day. This value type is defined by the following notation:

If the property permits, multiple "time" values are specified by a COMMA character (US-ASCII decimal 44) separated list of values. No additional content value encoding (i.e., BACKSLASH character encoding, see ) is defined for this value type. The "TIME" value type is used to identify values that contain a time of day. The format is based on the complete representation, basic format for a time of day. The text format consists of a two-digit 24-hour of the day (i.e., values 00-23), two-digit minute in the hour (i.e., values 00-59), and two-digit seconds in the minute (i.e., values 00-60). The seconds value of 60 MUST only be used to account for positive "leap" seconds. Fractions of a second are not supported by this format. In parallel to the "DATE-TIME" definition above, the "TIME" value type expresses time values in three forms: The form of time with UTC offset MUST NOT be used. For example, the following is not valid for a time value:

FORM #1 LOCAL TIME The local time form is simply a time value that does not contain the UTC designator nor does it reference a time zone. For example, 11:00 PM:

Time values of this type are said to be "floating" and are not bound to any time zone in particular. They are used to represent the same hour, minute, and second value regardless of which time zone is currently being observed. For example, an event can be defined that indicates that an individual will be busy from 11:00 AM to 1:00 PM every day, no matter which time zone the person is in. In these cases, a local time can be specified. The recipient of an iCalendar object with a property value consisting of a local time, without any relative time zone information, SHOULD interpret the value as being fixed to whatever time zone the "ATTENDEE" is in at any given moment. This means that two "Attendees", may participate in the same event at different UTC times; floating time SHOULD only be used where that is reasonable behavior. In most cases, a fixed time is desired. To properly communicate a fixed time in a property value, either UTC time or local time with time zone reference MUST be specified. The use of local time in a TIME value without the "TZID" property parameter is to be interpreted as floating time, regardless of the existence of "VTIMEZONE" calendar components in the iCalendar object. FORM #2: UTC TIME UTC time, or absolute time, is identified by a LATIN CAPITAL LETTER Z suffix character (US-ASCII decimal 90), the UTC designator, appended to the time value. For example, the following represents 07:00 AM UTC:

The "TZID" property parameter MUST NOT be applied to TIME properties whose time values are specified in UTC. FORM #3: LOCAL TIME AND TIME ZONE REFERENCE The local time with reference to time zone information form is identified by the use the "TZID" property parameter to reference the appropriate time zone definition. "TZID" is discussed in detail in . The following represents 8:30 AM in New York in Winter, five hours behind UTC, in each of the three formats:

URI This value type is used to identify values that contain a uniform resource identifier (URI) type of reference to the property value. This value type is defined by the following notation:

]]> This value type might be used to reference binary information, for values that are large, or otherwise undesirable to include directly in the iCalendar object. Property values with this value type MUST follow the generic URI syntax defined in . When a property parameter value is a URI value type, the URI MUST be specified as a quoted-string value. No additional content value encoding (i.e., BACKSLASH character encoding, see ) is defined for this value type. The following is a URI for a network file:

UTC-OFFSET This value type is used to identify properties that contain an offset from UTC to local time. This value type is defined by the following notation:

The PLUS SIGN (US-ASCII decimal 43) character MUST be specified for positive UTC offsets (i.e., ahead of UTC). The HYPHEN-MINUS character (US-ASCII decimal 45) MUST be specified for negative UTC offsets (i.e., behind of UTC). The value of "-0000" and "-000000" are not allowed. The time-second, if present, MUST NOT be 60; if absent, it defaults to zero. No additional content value encoding (i.e., BACKSLASH character encoding, see ) is defined for this value type. The following UTC offsets are given for standard time for New York (five hours behind UTC) and Geneva (one hour ahead of UTC):

The Calendaring and Scheduling Core Object is a collection of calendaring and scheduling information. Typically, this information will consist of an iCalendar stream with a single iCalendar object. However, multiple iCalendar objects can be sequentially grouped together in an iCalendar stream. The first line and last line of the iCalendar object MUST contain a pair of iCalendar object delimiter strings. The syntax for an iCalendar stream is as follows:

The following is a simple example of an iCalendar object:

A property is the definition of an individual attribute describing a calendar object or a calendar component. A property takes the form defined by the "contentline" notation defined in . The following is an example of a property:

This memo imposes no ordering of properties within an iCalendar object. Property names, parameter names and enumerated parameter values are case insensitive. For example, the property name "DUE" is the same as "due" and "Due", DTSTART;TZID=America/New_York:19980714T120000 is the same as DtStart;TzID=America/New_York:19980714T120000.

The body of the iCalendar object consists of a sequence of calendar properties and one or more calendar components. The calendar properties are attributes that apply to the calendar object as a whole. The calendar components are collections of properties that express a particular calendar semantic. For example, the calendar component can specify an event, a to-do, a journal entry, time zone information, free/busy time information, or an alarm. The body of the iCalendar object is defined by the following notation:

An iCalendar object MUST include the "PRODID" and "VERSION" calendar properties. In addition, it MUST include at least one calendar component. Special forms of iCalendar objects are possible to publish just busy time (i.e., only a "VFREEBUSY" calendar component) or time zone (i.e., only a "VTIMEZONE" calendar component) information. In addition, a complex iCalendar object that is used to capture a complete snapshot of the contents of a calendar is possible (e.g., composite of many different calendar components). More commonly, an iCalendar object will consist of just a single "VEVENT", "VTODO" or "VJOURNAL" calendar component. Applications MUST ignore x-comp and iana-comp they don't recognized. Applications that support importing iCalendar objects SHOULD support all of the component types defined in this document, and SHOULD NOT silently drop any components as that can lead to user data loss.

VEVENT Provide a grouping of component properties that describe an event. A "VEVENT" calendar component is defined by the following notation:

A "VEVENT" calendar component is a grouping of component properties, and possibly including "VALARM" calendar components, that represents a scheduled amount of time on a calendar. For example, it can be an activity; such as a one-hour long, department meeting from 8:00 AM to 9:00 AM, tomorrow. Generally, an event will take up time on an individual calendar. Hence, the event will appear as an opaque interval in a search for busy time. Alternately, the event can have its Time Transparency set to "TRANSPARENT" in order to prevent blocking of the event in searches for busy time. The "VEVENT" is also the calendar component used to specify an anniversary or daily reminder within a calendar. These events have a DATE value type for the "DTSTART" property instead of the default value type of DATE-TIME. If such a "VEVENT" has a "DTEND" property, it MUST be specified as a DATE value also. The anniversary type of "VEVENT" can span more than one date (i.e., "DTEND" property value is set to a calendar date after the "DTSTART" property value). If such a "VEVENT" has a "DURATION" property, it MUST be specified as a "dur-day" or "dur-week" value. The "DTSTART" property for a "VEVENT" specifies the inclusive start of the event. For recurring events, it also specifies the very first instance in the recurrence set. The "DTEND" property for a "VEVENT" calendar component specifies the non-inclusive end of the event. For cases where a "VEVENT" calendar component specifies a "DTSTART" property with a DATE value type but no "DTEND" nor "DURATION" property, the event's duration is taken to be one day. For cases where a "VEVENT" calendar component specifies a "DTSTART" property with a DATE-TIME value type but no "DTEND" property, the event ends on the same calendar date and time of day specified by the "DTSTART" property. The "VEVENT" calendar component cannot be nested within another calendar component. However, "VEVENT" calendar components can be related to each other or to a "VTODO" or to a "VJOURNAL" calendar component with the "RELATED-TO" property. The following is an example of the "VEVENT" calendar component used to represent a meeting that will also be opaque to searches for busy time:

The following is an example of the "VEVENT" calendar component used to represent a reminder that will not be opaque, but rather transparent, to searches for busy time:

The following is an example of the "VEVENT" calendar component used to represent an anniversary that will occur annually.

The following is an example of the "VEVENT" calendar component used to represent a multi-day event scheduled from June 28th, 2007 to July 8th, 2007 inclusively. Note that the "DTEND" property is set to July 9th, 2007, since the "DTEND" property specifies the non-inclusive end of the event.

VTODO Provide a grouping of calendar properties that describe a to-do. A "VTODO" calendar component is defined by the following notation:

A "VTODO" calendar component is a grouping of component properties and possibly "VALARM" calendar components that represent an action-item or assignment. For example, it can be used to represent an item of work assigned to an individual; such as "turn in travel expense today". The "VTODO" calendar component cannot be nested within another calendar component. However, "VTODO" calendar components can be related to each other or to a "VEVENT" or to a "VJOURNAL" calendar component with the "RELATED-TO" property. A "VTODO" calendar component without the "DTSTART" and "DUE" (or "DURATION") properties specifies a to-do that will be associated with each successive calendar date, until it is completed. The following is an example of a "VTODO" calendar component that needs to be completed before May 1st, 2007. On midnight May 1st, 2007 this to-do would be considered overdue.

The following is an example of a "VTODO" calendar component that was due before 1:00 P.M. UTC on July 9th, 2007 and was completed on July 7th, 2007 at 10:00 A.M. UTC.

VJOURNAL Provide a grouping of component properties that describe a journal entry. A "VJOURNAL" calendar component is defined by the following notation:

A "VJOURNAL" calendar component is a grouping of component properties that represent one or more descriptive text notes associated with a particular calendar date. The "DTSTART" property is used to specify the calendar date that the journal entry is associated with. Generally, it will have a DATE value data type, but it can also be used to specify a DATE-TIME value data type. Examples of a journal entry include a daily record of a legislative body or a journal entry of individual telephone contacts for the day or an ordered list of accomplishments for the day. The "VJOURNAL" calendar component can also be used to associate a document with a calendar date. The "VJOURNAL" calendar component does not take up time on a calendar. Hence, it does not play a role in free or busy time searches -- it is as though it has a time transparency value of TRANSPARENT. It is transparent to any such searches. The "VJOURNAL" calendar component cannot be nested within another calendar component. However, "VJOURNAL" calendar components can be related to each other or to a "VEVENT" or to a "VTODO" calendar component, with the "RELATED-TO" property. The following is an example of the "VJOURNAL" calendar component:

VFREEBUSY Provide a grouping of component properties that describe either a request for free/busy time, describe a response to a request for free/busy time or describe a published set of busy time. A "VFREEBUSY" calendar component is defined by the following notation:

A "VFREEBUSY" calendar component is a grouping of component properties that represents either a request for, a reply to a request for free or busy time information or a published set of busy time information. When used to request free/busy time information, the "ATTENDEE" property specifies the calendar users whose free/busy time is being requested; the "ORGANIZER" property specifies the calendar user who is requesting the free/busy time; the "DTSTART" and "DTEND" properties specify the window of time for which the free/busy time is being requested; the "UID" and "DTSTAMP" properties are specified to assist in proper sequencing of multiple free/busy time requests. When used to reply to a request for free/busy time, the "ATTENDEE" property specifies the calendar user responding to the free/busy time request; the "ORGANIZER" property specifies the calendar user that originally requested the free/busy time; the "FREEBUSY" property specifies the free/busy time information (if it exists); and the "UID" and "DTSTAMP" properties are specified to assist in proper sequencing of multiple free/busy time replies. When used to publish busy time, the "ORGANIZER" property specifies the calendar user associated with the published busy time; the "DTSTART" and "DTEND" properties specify an inclusive time window that surrounds the busy time information; the "FREEBUSY" property specifies the published busy time information; and the "DTSTAMP" property specifies the date/time that iCalendar object was created. The "VFREEBUSY" calendar component cannot be nested within another calendar component. Multiple "VFREEBUSY" calendar components can be specified within an iCalendar object. This permits the grouping of Free/Busy information into logical collections, such as monthly groups of busy time information. The "VFREEBUSY" calendar component is intended for use in iCalendar object methods involving requests for free time, requests for busy time, requests for both free and busy, and the associated replies. Free/Busy information is represented with the "FREEBUSY" property. This property provides a terse representation of time periods. One or more "FREEBUSY" properties can be specified in the "VFREEBUSY" calendar component. When present in a "VFREEBUSY" calendar component, the "DTSTART" and "DTEND" properties SHOULD be specified prior to any "FREEBUSY" properties. The recurrence properties ("RRULE", "RDATE", "EXDATE") are not permitted within a "VFREEBUSY" calendar component. Any recurring events are resolved into their individual busy time periods using the "FREEBUSY" property. The following is an example of a "VFREEBUSY" calendar component used to request free or busy time information:

The following is an example of a "VFREEBUSY" calendar component used to reply to the request with busy time information:

The following is an example of a "VFREEBUSY" calendar component used to publish busy time information.

VTIMEZONE Provide a grouping of component properties that defines a time zone. A "VTIMEZONE" calendar component is defined by the following notation:

A time zone is unambiguously defined by the set of time measurement rules determined by the governing body for a given geographic area. These rules describe at a minimum the base offset from UTC for the time zone, often referred to as the Standard Time offset. Many locations adjust their Standard Time forward or backward by one hour, in order to accommodate seasonal changes in number of daylight hours, often referred to as Daylight Saving Time. Some locations adjust their time by a fraction of an hour. Standard Time is also known as Winter Time. Daylight Saving Time is also known as Advanced Time, Summer Time, or Legal Time in certain countries. The following table shows the changes in time zone rules in effect for New York City starting from 1967. Each line represents a description or rule for a particular observance. Effective Observance Rule Date (Date/Time) Offset Abbreviation 1967-1973 last Sun in Apr, 02:00 -0400 EDT 1967-2006 last Sun in Oct, 02:00 -0500 EST 1974-1974 Jan 6, 02:00 -0400 EDT 1975-1975 Feb 23, 02:00 -0400 EDT 1976-1986 last Sun in Apr, 02:00 -0400 EDT 1987-2006 first Sun in Apr, 02:00 -0400 EDT 2007-* second Sun in Mar, 02:00 -0400 EDT 2007-* first Sun in Nov, 02:00 -0500 EST Note: The specification of a global time zone registry is not addressed by this document and is left for future study. However, implementers may find the TZ database a useful reference. It is an informal, public-domain collection of time zone information, which is currently being maintained by volunteer Internet participants, and is used in several operating systems. This database contains current and historical time zone information for a wide variety of locations around the globe; it provides a time zone identifier for every unique time zone rule set in actual use since 1970, with historical data going back to the introduction of standard time. Interoperability between two calendaring and scheduling applications, especially for recurring events, to-dos or journal entries, is dependent on the ability to capture and convey date and time information in an unambiguous format. The specification of current time zone information is integral to this behavior. If present, the "VTIMEZONE" calendar component defines the set of Standard Time and Daylight Saving Time observances (or rules) for a particular time zone for a given interval of time. The "VTIMEZONE" calendar component cannot be nested within other calendar components. Multiple "VTIMEZONE" calendar components can exist in an iCalendar object. In this situation, each "VTIMEZONE" MUST represent a unique time zone definition. This is necessary for some classes of events, such as airline flights, that start in one time zone and end in another. The "VTIMEZONE" calendar component MUST include the "TZID" property and at least one definition of a "STANDARD" or "DAYLIGHT" sub-component. The "STANDARD" or "DAYLIGHT" subcomponent MUST include the "DTSTART", "TZOFFSETFROM" and "TZOFFSETTO" properties. An individual "VTIMEZONE" calendar component MUST be specified for each unique "TZID" parameter value specified in the iCalendar object. In addition, a "VTIMEZONE" calendar component, referred to by a recurring calendar component, MUST provide valid time zone information for all recurrence instances. Each "VTIMEZONE" calendar component consists of a collection of one or more sub-components that describe the rule for a particular observance (either a Standard Time or a Daylight Saving Time observance). The "STANDARD" sub-component consists of a collection of properties that describe Standard Time. The "DAYLIGHT" sub-component consists of a collection of properties that describe Daylight Saving Time. In general this collection of properties consists of: the first onset date-time for the observance; the last onset date-time for the observance, if a last onset is known; the offset to be applied for the observance; a rule that describes the day and time when the observance takes effect; an optional name for the observance. For a given time zone, there may be multiple unique definitions of the observances over a period of time. Each observance is described using either a "STANDARD" or "DAYLIGHT" sub-component. The collection of these sub-components is used to describe the time zone for a given period of time. The offset to apply at any given time is found by locating the observance that has the last onset date and time before the time in question, and using the offset value from that observance. The top-level properties in a "VTIMEZONE" calendar component are: The mandatory "TZID" property is a text value that uniquely identifies the "VTIMEZONE" calendar component within the scope of an iCalendar object. The optional "LAST-MODIFIED" property is a UTC value that specifies the date and time that this time zone definition was last updated. The optional "TZURL" property is a url value that points to a published "VTIMEZONE" definition. "TZURL" SHOULD refer to a resource that is accessible by anyone who might need to interpret the object. This SHOULD NOT normally be a "file" URL or other URL that is not widely-accessible. The collection of properties that are used to define the "STANDARD" and "DAYLIGHT" sub-components include: The mandatory "DTSTART" property gives the effective onset date and local time for the time zone sub-component definition. "DTSTART" in this usage MUST be specified as a date with local time value. The mandatory "TZOFFSETFROM" property gives the UTC offset which is in use when the onset of this time zone observance begins. "TZOFFSETFROM" is combined with "DTSTART" to define the effective onset for the time zone sub-component definition. For example, the following represents the time at which the observance of Standard Time took effect in Fall 1967 for New York City:

The mandatory "TZOFFSETTO" property gives the UTC offset for the time zone sub-component (Standard Time or Daylight Saving Time) when this observance is in use. The optional "TZNAME" property is the customary name for the time zone. This could be used for displaying dates. The onset date-times for the observance defined by the time zone sub-component is defined by the "DTSTART", "RRULE", and "RDATE" properties. The "RRULE" property defines the recurrence rule for the onset of the observance defined by this time zone sub-component. Some specific requirements for the usage of "RRULE" for this purpose include: If observance is known to have an effective end date, the "UNTIL" recurrence rule parameter MUST be used to specify the last valid onset of this observance (i.e., the UNTIL date-time will be equal to the last instance generated by the recurrence pattern). It MUST be specified in UTC time. The "DTSTART" and the "TZOFFSETFROM" properties MUST be used when generating the onset date-time values (instances) from the "RRULE". The "RDATE" property can also be used to define the onset of the observance by giving the individual onset date and times. "RDATE" in this usage MUST be specified as a date with local time value, relative to the UTC offset specified in the "TZOFFSETFROM" property. The optional "COMMENT" property is also allowed for descriptive explanatory text. The following are examples of the "VTIMEZONE" calendar component: This is an example showing all the time zone rules for New York City since April 30, 1967 at 03:00:00 EDT.

This is an example showing time zone information for New York City using only the "DTSTART" property. Note that this is only suitable for a recurring event that starts on or later than March 11, 2007 at 03:00:00 EDT (i.e., the earliest effective transition date and time) and ends no later than March 9, 2008 at 01:59:59 EST (i.e., latest valid date and time for EST in this scenario). For example, this can be used for a recurring event that occurs every Friday, 8:00 A.M.-9:00 A.M., starting June 1, 2007, ending December 31, 2007,

This is a simple example showing the current time zone rules for New York City using a "RRULE" recurrence pattern. Note that there is no effective end date to either of the Standard Time or Daylight Time rules. This information would be valid for a recurring event starting today and continuing indefinitely.

This is an example showing a set of rules for a fictitious time zone where the Daylight Time rule has an effective end date (i.e., after that date, Daylight Time is no longer observed).

This is an example showing a set of rules for a fictitious time zone where the first Daylight Time rule has an effective end date. There is a second Daylight Time rule that picks up where the other left off.

VALARM Provide a grouping of component properties that define an alarm. A "VALARM" calendar component is defined by the following notation:

A "VALARM" calendar component is a grouping of component properties that is a reminder or alarm for an event or a to-do. For example, it may be used to define a reminder for a pending event or an overdue to-do. The "VALARM" calendar component MUST include the "ACTION" and "TRIGGER" properties. The "ACTION" property further constrains the "VALARM" calendar component in the following ways: When the action is "AUDIO", the alarm can also include one and only one "ATTACH" property, which MUST point to a sound resource, which is rendered when the alarm is triggered. When the action is "DISPLAY", the alarm MUST also include a "DESCRIPTION" property, which contains the text to be displayed when the alarm is triggered. When the action is "EMAIL", the alarm MUST include a "DESCRIPTION" property, which contains the text to be used as the message body, a "SUMMARY" property, which contains the text to be used as the message subject, and one or more "ATTENDEE" properties, which contain the email address of attendees to receive the message. It can also include one or more "ATTACH" properties, which are intended to be sent as message attachments. When the alarm is triggered, the email message is sent. The "VALARM" calendar component MUST only appear within either a "VEVENT" or "VTODO" calendar component. "VALARM" calendar components cannot be nested. Multiple mutually independent "VALARM" calendar components can be specified for a single "VEVENT" or "VTODO" calendar component. The "TRIGGER" property specifies when the alarm will be triggered. The "TRIGGER" property specifies a duration prior to the start of an event or a to-do. The "TRIGGER" edge may be explicitly set to be relative to the "START" or "END" of the event or to-do with the "RELATED" parameter of the "TRIGGER" property. The "TRIGGER" property value type can alternatively be set to an absolute calendar date with UTC time. In an alarm set to trigger on the "START" of an event or to-do, the "DTSTART" property MUST be present in the associated event or to-do. In an alarm in a "VEVENT" calendar component set to trigger on the "END" of the event, either the "DTEND" property MUST be present, or the "DTSTART" and "DURATION" properties MUST both be present. In an alarm in a "VTODO" calendar component set to trigger on the "END" of the to-do, either the "DUE" property MUST be present, or the "DTSTART" and "DURATION" properties MUST both be present. The alarm can be defined such that it triggers repeatedly. A definition of an alarm with a repeating trigger MUST include both the "DURATION" and "REPEAT" properties. The "DURATION" property specifies the delay period, after which the alarm will repeat. The "REPEAT" property specifies the number of additional repetitions that the alarm will be triggered. This repetition count is in addition to the initial triggering of the alarm. Both of these properties MUST be present in order to specify a repeating alarm. If one of these two properties is absent, then the alarm will not repeat beyond the initial trigger. The "ACTION" property is used within the "VALARM" calendar component to specify the type of action invoked when the alarm is triggered. The "VALARM" properties provide enough information for a specific action to be invoked. It is typically the responsibility of a "Calendar User Agent" (CUA) to deliver the alarm in the specified fashion. An "ACTION" property value of AUDIO specifies an alarm that causes a sound to be played to alert the user; DISPLAY specifies an alarm that causes a text message to be displayed to the user; and EMAIL specifies an alarm that causes an electronic email message to be delivered to one or more email addresses. In an AUDIO alarm, if the optional "ATTACH" property is included, it MUST specify an audio sound resource. The intention is that the sound will be played as the alarm effect. If an "ATTACH" property is specified that does not refer to a sound resource, or if the specified sound resource cannot be rendered (because its format is unsupported, or because it cannot be retrieved), then the CUA or other entity responsible for playing the sound may choose a fallback action, such as playing a built-in default sound, or playing no sound at all. In a DISPLAY alarm, the intended alarm effect is for the text value of the "DESCRIPTION" property to be displayed to the user. In an EMAIL alarm, the intended alarm effect is for an email message to be composed and delivered to all the addresses specified by the "ATTENDEE" properties in the "VALARM" calendar component. The "DESCRIPTION" property of the "VALARM" calendar component MUST be used as the body text of the message, and the "SUMMARY" property MUST be used as the subject text. Any "ATTACH" properties in the "VALARM" calendar component SHOULD be sent as attachments to the message. Note: Implementations should carefully consider whether they accept alarm components from untrusted sources, e.g., when importing calendar objects from external sources. One reasonable policy is to always ignore alarm components that the calendar user has not set herself, or at least ask for confirmation in such a case. The following example is for a "VALARM" calendar component that specifies an audio alarm that will sound at a precise time and repeat 4 more times at 15 minute intervals:

The following example is for a "VALARM" calendar component that specifies a display alarm that will trigger 30 minutes before the scheduled start of the event or of the to-do it is associated with and will repeat 2 more times at 15 minute intervals:

The following example is for a "VALARM" calendar component that specifies an email alarm that will trigger 2 days before the scheduled due date/time of a to-do it is associated with. It does not repeat. The email has a subject, body and attachment link.

The Calendar Properties are attributes that apply to the iCalendar object, as a whole. These properties do not appear within a calendar component. They SHOULD be specified after the "BEGIN:VCALENDAR" delimiter string and prior to any calendar component.

CALSCALE This property defines the calendar scale used for the calendar information specified in the iCalendar object. TEXT IANA and non-standard property parameters can be specified on this property. This property can be specified once in an iCalendar object. The default value is "GREGORIAN". This memo is based on the Gregorian calendar scale. The Gregorian calendar scale is assumed if this property is not specified in the iCalendar object. It is expected that other calendar scales will be defined in other specifications or by future versions of this memo. This property is defined by the following notation:

The following is an example of this property:

METHOD This property defines the iCalendar object method associated with the calendar object. TEXT IANA and non-standard property parameters can be specified on this property. This property can be specified once in an iCalendar object. When used in a MIME message entity, the value of this property MUST be the same as the Content-Type "method" parameter value. If either the "METHOD" property or the Content-Type "method" parameter is specified, then the other MUST also be specified. No methods are defined by this specification. This is the subject of other specifications, such as the iCalendar Transport-independent Interoperability Protocol (iTIP) defined by . If this property is not present in the iCalendar object, then a scheduling transaction MUST NOT be assumed. In such cases, the iCalendar object is merely being used to transport a snapshot of some calendar information; without the intention of conveying a scheduling semantic. This property is defined by the following notation:

The following is a hypothetical example of this property to convey that the iCalendar object is a scheduling request:

PRODID This property specifies the identifier for the product that created the iCalendar object. TEXT IANA and non-standard property parameters can be specified on this property. The property MUST be specified once in an iCalendar object. The vendor of the implementation SHOULD assure that this is a globally unique identifier; using some technique such as an FPI value, as defined in . This property SHOULD NOT be used to alter the interpretation of an iCalendar object beyond the semantics specified in this memo. For example, it is not to be used to further the understanding of non-standard properties. This property is defined by the following notation:

The following is an example of this property. It does not imply that English is the default language.

VERSION This property specifies the identifier corresponding to the highest version number or the minimum and maximum range of the iCalendar specification that is required in order to interpret the iCalendar object. TEXT IANA and non-standard property parameters can be specified on this property. This property MUST be specified once in an iCalendar object. A value of "2.0" corresponds to this memo. This property is defined by the following notation:

;Minimum iCalendar version needed to parse the iCalendar object maxver = ;Maximum iCalendar version needed to parse the iCalendar object ]]> The following is an example of this property:

The following properties can appear within calendar components, as specified by each component property definition.

The following properties specify descriptive information about calendar components.

ATTACH This property provides the capability to associate a document object with a calendar component. The default value type for this property is URI. The value type can also be set to BINARY to indicate inline binary encoded content information. IANA, non-standard, inline encoding and value data type property parameters can be specified on this property. The format type parameter can be specified on this property and is RECOMMENDED for inline binary encoded content information. This property can be specified multiple times in a "VEVENT", "VTODO", "VJOURNAL" or "VALARM" calendar component with the exception of AUDIO alarm which only allows this property to occur once . This property is used in "VEVENT", "VTODO", and "VJOURNAL" calendar components to associate a resource (e.g., document) with the calendar component. This property is used in "VALARM" calendar components to specify an audio sound resource or an email message attachment. This property can be specified as a URI pointing to a resource or as inline binary encoded content. When this property is specified as inline binary encoded content, calendar applications MAY attempt to guess the media type of the resource via inspection of its content if and only if the media type of the resource is not given by the "FMTTYPE" parameter. If the media type remains unknown, calendar applications SHOULD treat it as type "application/octet-stream". This property is defined by the following notation:

The following are examples of this property: