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ART Independent Submission time zone tzdata tzif This document specifies the Time Zone Information Format (TZif) for representing and exchanging time zone information, independent of any particular service or protocol. Two MIME media types for this format are also defined.

Time zone data typically consists of offsets from Universal Time (UT), daylight saving transition rules, one or more local time designations (acronyms or abbreviations), and optional leap second adjustments. One such format for conveying this information is iCalendar. It is a text-based format used by calendaring and scheduling systems. This document specifies the widely deployed Time Zone Information Format (TZif). It is a binary format used by most UNIX systems to calculate local time. This format was introduced in the 1980s and has evolved since then into multiple upward-compatible versions. There is a wide variety of interoperable software capable of generating and reading files in this format. This specification does not define the source of the data assembled into a TZif file. One such source is the IANA-hosted time zone database .

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 when, and only when, they appear in all capitals, as shown here. The following terms are used in this document (see "Sources for Time Zone and Daylight Saving Time Data" for more-detailed information about civil timekeeping data and practice): The basis for civil time since 1960. It is approximately equal to mean solar time at the prime meridian (0 degrees longitude). The time according to a location's law or practice, when adjusted as necessary from standard time. The adjustment may be positive or negative, and the amount of adjustment may vary depending on the date and time; the TZif format even allows the adjustment to be zero, although this is not common practice. The time standard based on atomic clocks since 1972. It is equal to UTC except without leap second adjustments. The value of TAI - UTC - 10 for timestamps after the first leap second, and zero for timestamps before that. The expression "TAI - UTC - 10" comes from the fact that TAI - UTC was defined to be 10 just prior to the first leap second in 1972, so clocks with leap seconds have a zero LEAPCORR before the first leap second. Civil time for a particular location. Its offset from Universal Time can depend on the date and time of day. 1970-01-01 00:00:00 UTC, the basis for absolute timestamps in this document. The time according to a location's law or practice, unadjusted for Daylight Saving Time. A change to civil timekeeping practice. It occurs when one or more of the following happen simultaneously: a change in UT offset a change in whether daylight saving time is in effect a change in time zone abbreviation a leap second (i.e., a change in LEAPCORR) The Time Zone Data Distribution Service (TZDIST) defines "Time zone data" as "data that defines a single time zone, including an identifier, UTC offset values, DST rules, and other information such as time zone abbreviations." The interchange format defined in this document is one such form of time zone data. The moment of occurrence of a time change that is not a leap second. It is identified with a signed integer count of Unix Leap Time seconds since the POSIX Epoch. The basis of civil time. This is the principal form of the mean solar time at the prime meridian (0 degrees longitude) for timestamps before UTC was introduced in 1960, and is UTC for timestamps thereafter. Although UT is sometimes called "UTC" or "GMT" in other sources, this specification uses the term "UT" to avoid confusion with UTC or with GMT. The time as returned by the C time() function (see Section 3 of the "System Interfaces" Volume of ). This is an integer number of seconds since the POSIX Epoch, not counting leap seconds. As an extension to POSIX, negative values represent times before the POSIX Epoch, using UT. UNIX time plus all preceding leap second corrections. For example, if the first leap second record in a TZif file occurs at 1972-06-30 23:59:60 UTC, the UNIX leap time for the timestamp 1972-07-01 00:00:00 UTC would be 78796801, one greater than the UNIX time for the same timestamp. Similarly, if the second leap second record occurs at 1972-12-31 23:59:60 UTC it accounts for the first leap second, so the UNIX leap time of 1972-12-31 23:59:60 UTC would be 94694401 and the Unix leap time of 1973-01-01 00:00:00 UTC would be 94694402. If a TZif file specifies no leap second records, UNIX leap time is equal to UNIX time. Another name for local time; short for "wall clock time".

The time zone information format begins with a fixed 44-octet version 1 header containing a field that specifies the version of the file's format. Readers designed for version N can read version N+1 files without too much trouble; data specific to version N+1 either appears after version N data so that earlier-version readers can easily ignore later-version data they are not designed for, or it appears as a minor extension to version N that version N readers are likely to tolerate well. The version 1 header is followed by a variable-length version 1 data block containing four-octet (32-bit) transition times and leap second occurrences. These 32-bit values are limited to representing time changes from 1901-12-13 20:45:52 through 2038-01-19 03:14:07 UT, and the version 1 header and data block are present only for backward compatibility with obsolescent readers as discussed in Common Interoperability Issues. Version 1 files terminate after the version 1 data block. Version 2 and 3 files extend the format by appending a second 44-octet version 2+ header, a variable-length version 2+ data block containing eight-octet (64-bit) transition times and leap second occurrences, and a variable length footer. These 64-bit values can represent times approximately 292 billion years into the past or future. NOTE: All multi-octet integer values MUST be stored in network octet order format (high-order octet first, otherwise known as big-endian), with all bits significant. Signed integer values MUST be represented using two's complement. A TZif file is structured as follows:

A TZif header is structured as follows (the lengths of multi-octet fields are shown in parentheses):

The fields of the header are defined as follows: The four-octet ASCII sequence "TZif" (0x54 0x5A 0x69 0x66) which identifies the file as utilizing the Time Zone Information Format. An octet identifying the version of the file's format. The value MUST be one of the following: Version 1 - The file contains only the version 1 header and data block. Version 1 files MUST NOT contain a version 2+ header, data block, or footer. Version 2 - The file MUST contain the version 1 header and data block, a version 2+ header and data block, and a footer. The TZ string in the footer, if nonempty, MUST strictly adhere to the requirements for the TZ environment variable as defined in Section 8.3 of the "Base Definitions" Volume of , and MUST encode the POSIX portable character set as ASCII. Version 3 - The file MUST contain the version 1 header and data block, a version 2+ header and data block, and a footer. The TZ string in the footer, if nonempty, MUST conform to POSIX requirements with ASCII encoding, except that it MAY use the TZ string extensions described below. A four-octet unsigned integer specifying the number of UT/local indicators contained in the data block - MUST either be zero or equal to 'typecnt'. A four-octet unsigned integer specifying the number of standard/wall indicators contained in the data block - MUST either be zero or equal to 'typecnt'. A four-octet unsigned integer specifying the number of leap second records contained in the data block. A four-octet unsigned integer specifying the number of transition times contained in the data block. A four-octet unsigned integer specifying the number of local time type records contained in the data block - MUST NOT be zero. (Although local time type records convey no useful information in files that have nonempty TZ strings but no transitions, at least one such record is nevertheless required because many TZif readers reject files that have zero time types.) A four-octet unsigned integer specifying the total number of octets used by the set of time zone designations contained in the data block - MUST NOT be zero. The count includes the trailing NUL (0x00) octet at the end of the last time zone designation. Although the version 1 and 2+ headers have the same format with the same magic number and version fields, their count fields may differ because the version 1 data can be a subset of the version 2+ data.

A TZif data block consists of seven variable-length elements, each of which is a series of items. The number of items in each series is determined by the corresponding count field in the header. The total length of each element is calculated by multiplying the number of items by the size of each item. Therefore, implementations that do not wish to parse or use the version 1 data block can calculate its total length and skip directly to the header of the version 2+ data block. In the version 1 data block, time values are 32-bit (TIME_SIZE = 4 octets). In the version 2+ data block, present only in version 2 and 3 files, time values are 64-bit (TIME_SIZE = 8 octets). The data block is structured as follows (the lengths of multi-octet fields are shown in parentheses):

The elements of the data block are defined as follows: A series of four- or eight-octet UNIX leap time values sorted in strictly ascending order. Each value is used as a transition time at which the rules for computing local time may change. The number of time values is specified by the 'timecnt' field in the header. Each time value SHOULD be at least -2**59. (-2**59 is the greatest negated power of 2 that predates the Big Bang, and avoiding earlier timestamps works around known TZif reader bugs relating to outlandlishly negative timestamps.) A series of one-octet unsigned integers specifying the type of local time of the corresponding transition time. These values serve as zero-based indices into the array of local time type records. The number of type indices is specified by the 'timecnt' field in the header. Each type index MUST be in the range [0, 'typecnt' - 1]. A series of six-octet records specifying a local time type. The number of records is specified by the 'typecnt' field in the header. Each record has the following format (the lengths of multi-octet fields are shown in parentheses):

A four-octet signed integer specifying the number of seconds to be added to UT in order to determine local time. The value MUST NOT be -2**31, and SHOULD be in the range [-89999, 93599] (i.e., its value SHOULD be more than -25 hours and less than 26 hours). (Avoiding -2**31 allows 32-bit clients to negate the value without overflow. Restricting it to [-89999, 93599] allows easy support by implementations that already support the the POSIX-required range [-24:59:59, 25:59:59].) A one-octet value indicating whether local time should be considered Daylight Saving Time (DST). The value MUST be 0 or 1. A value of one (1) indicates that this type time is DST. A value of zero (0) indicates that this time type is standard time. A one-octet unsigned integer specifying a zero-based index into the series of time zone designation octets, thereby selecting a particular designation string. Each index MUST be in the range [0, 'charcnt' - 1], and designates the NUL-terminated string of octets starting at position 'idx' in the time zone designations. (This string MAY be empty.) A NUL octet MUST exist in the time zone designations at or after position 'idx'. A series of octets constituting an array of NUL-terminated (0x00) time zone designation strings. The total number of octets is specified by the 'charcnt' field in the header. Note that two designations MAY overlap if one is a suffix of the other. The character encoding of time zone designation strings is not specified; however, see of this document. A series of eight- or twelve-octet records specifying the corrections that need to be applied to UTC in order to determine TAI. The records are sorted by the occurrence time in strictly ascending order. The number of records is specified by the 'leapcnt' field in the header. Each record has one of the following structures (the lengths of multi-octet fields are shown in parentheses):

A four- or eight-octet UNIX leap time value specifying the time at which a leap second correction occurs. The first value, if present, MUST be nonnegative, and each later value MUST be at least 2419199 greater than the previous value. (This is 28 days' worth of seconds, minus a potential negative leap second.) A four-octet signed integer specifying the value of LEAPCORR on or after the occurrence. The correction value in the first leap second record, if present, MUST be either one (1) or minus one (-1). The correction values in adjacent leap second records MUST differ by exactly one (1). The value of LEAPCORR is zero for timestamps that occur before the occurrence time in the first leap second record (or for all timestamps if there are no leap second records). A series of one-octet values indicating whether the transition times associated with local time types were specified as standard time or wall clock time. Each value MUST be 0 or 1. A value of one (1) indicates standard time, and MUST be set to one (1) if the corresponding UT/local indicator is set to one (1). A value of zero (0) indicates wall time. The number of values is specified by the 'isstdcnt' field in the header. If 'isstdcnt' is zero (0), all transition times associated with local time types are assumed to be specified as wall time. A series of one-octet values indicating whether the transition times associated with local time types were specified as UT or local time. Each value MUST be 0 or 1. A value of one (1) indicates UT, and the corresponding standard/wall indicator MUST also be set to one (1). A value of zero (0) indicates local time. The number of values is specified by the 'isutcnt' field in the header. If 'isutcnt' is zero (0), all transition times associated with local time types are assumed to be specified as local time. The type corresponding to a transition time specifies local time for timestamps starting at the given transition time and continuing up to, but not including, the next transition time. Local time for timestamps before the first transition is specified by the first time type (time type 0). Local time for timestamps on or after the last transition is specified by the TZ string in the footer if present and nonempty, and is unspecified otherwise. If there are no transitions, local time for all timestamps is specified by the TZ string in the footer if present and nonempty, and is specified by time type 0 otherwise. A given pair of standard/wall and UT/local indicators is used to designate whether the corresponding transition time was specified as UT, standard time, or wall clock time. Note that there are only three combinations of the two indicators given that the standard/wall value MUST be one (1) if the UT/local value is one (1). This information can be useful if the transition times in a TZif file need to be transformed into transitions appropriate for another time zone (e.g. when calculating transition times for a simple POSIX TZ string such as "AKST9AKDT"). In order to eliminate unused space in a TZif file, every nonzero local time type index SHOULD appear at least once in the transition type array. Likewise, every octet in the time zone designations array SHOULD be used by at least one time type record.

The TZif footer is structured as follows (the lengths of multi-octet fields are shown in parentheses):

The elements of the footer are defined as follows: An ASCII new line character (0x0A). A rule for computing local time changes after the last transition time stored in the version 2+ data block. The string is either empty or uses the expanded format of the "TZ" environment variable as defined in Section 8.3 of the "Base Definitions" Volume of with ASCII encoding, possibly utilizing extensions described below in version 3 files. If the string is empty, the corresponding information is not available. If the string is nonempty and one or more transitions appear in the version 2+ data, the string MUST be consistent with the last version 2+ transition - i.e., evaluating the TZ string at the time of the last transition should yield the same time type as the time type specified in the last transition. The string MUST NOT contain NUL octets or be NUL-terminated, and SHOULD NOT begin with the ':' (colon) character. The TZif footer is present only in Version 2 and 3 files, as the obsolescent Version 1 format was designed before the need for a footer was apparent.

The TZ string in a Version 3 TZif file MAY use the following extensions to POSIX TZ strings. These extensions are described using the terminology of Section 8.3 of the "Base Definitions" Volume of . The hours part of the transition times may be signed and range from -167 through 167 (-167 <= hh <= 167) instead of the POSIX-required unsigned values from 0 through 24. This represents a time zone that observes daylight saving time from 22:00 on the day before March's last Sunday until 23:00 on the day before October's last Sunday. Standard time is 3 hours west of UT and is abbreviated "-03"; daylight saving time is 2 hours west of UT and is abbreviated "-02". DST is considered to be in effect all year if it starts January 1 at 00:00 and ends December 31 at 24:00 plus the difference between daylight saving and standard time, leaving no room for standard time in the calendar. This represents a time zone that observes daylight saving time all year. It is 4 hours west of UT and is abbreviated "EDT".

The following practices help ensure interoperability of TZif applications. Version 1 files are considered a legacy format and SHOULD NOT be generated, as they do not support transition times after the year 2038. Implementations that only understand Version 1 MUST ignore any data that extends beyond the calculated end of the version 1 data block. Implementations SHOULD generate a version 3 file if TZ string extensions are necessary to accurately model transition times. Otherwise, version 2 files SHOULD be generated. The sequence of time changes defined by the version 1 header and data block SHOULD be a contiguous subsequence of the time changes defined by the version 2+ header and data block, and by the footer. This guideline helps obsolescent version 1 readers agree with current readers about timestamps within the contiguous subsequence. It also lets writers not supporting obsolescent readers use a 'timecnt' of zero in the version 1 data block to save space. Time zone designations SHOULD consist of at least three (3) and no more than six (6) ASCII characters from the set of alphanumerics, '-', and '+'. This is for compatibility with POSIX requirements for time zone abbreviations. When reading a version 2 or 3 file, implementations SHOULD ignore the version 1 header and data block except for the purpose of skipping over them. Implementations SHOULD calculate the total lengths of the headers and data blocks and check that they all fit within the actual file size, as part of a validity check for the file. When a TZif file is used in a MIME message entity it SHOULD be indicated by one of the following media types: "application/tzif-leap" to indicate that leap second records are included in the TZif data as necessary (none are necessary if the file is truncated to a range that precedes the first leap second). "application/tzif" to indicate that leap second records are not included in the TZif data; 'leapcnt' in the header(s) MUST be zero (0). Common interoperability issues and possible workarounds are described in .

The Time Zone Data Distribution Service (TZDIST) is a service that allows reliable, secure, and fast delivery of time zone data and leap second rules to client systems such as calendaring and scheduling applications or operating systems. A TZDIST service MAY supply time zone data to clients in the Time Zone Information Format. Such a service MUST indicate that it supports this format by including the MIME media type "application/tzif" in its "capabilities" response (see Section 5.1 of ). A TZDIST service MAY also include the MIME media type "application/tzif-leap" in its "capabilities" response if it is able to generate TZif files containing leap second records. A TZDIST service MUST NOT advertise the "application/tzif-leap" MIME media type without also advertising "application/tzif". TZDIST clients MUST use the HTTP "Accept" header field to indicate their preference to receive data in the "application/tzif" and/or "application/tzif-leap" formats.

As described in Section 3.9 of , a TZDIST service MAY truncate time zone transition data. A truncated TZif file is valid from its first and up to, but not including, its last version 2+ transition time, if present. When truncating the start of a TZif file, the service MUST supply in the version 2+ data a first transition time that is the start point of the truncation range. As with untruncated TZif files, time type 0 indicates local time immediately before the start point, and the time type of the first transition indicates local time thereafter. When truncating the end of a TZif file, the service MUST supply in the version 2+ data a last transition time that is the end point of the truncation range, and MUST supply an empty TZ string. As with untruncated TZif files with empty TZ strings, a truncated TZif file does not indicate local time after the last transition. All represented information that falls inside the truncation range MUST be the same as that represented by a corresponding untruncated TZif file. TZDIST clients SHOULD NOT use a truncated TZif file (as described above) to interpret timestamps outside the truncation time range.

In this example, the client checks the server for the available formats and then requests that the time zone with a specific time zone identifier be returned in Time Zone Information Format.

Note that this example presumes that the time zone context path has been discovered (see Section 4.2.1) to be "/tzdist". > Request << GET /tzdist/capabilities HTTP/1.1 Host: tz.example.com >> Response << HTTP/1.1 200 OK Date: Fri, 01 Jun 2018 14:52:23 GMT Content-Type: application/json; charset="utf-8" Content-Length: xxxx { "version": 1, "info": { "primary-source": "IANA:2018e", "formats": [ "text/calendar", "application/tzif", "application/tzif-leap" ], ... }, ... } >> Request << GET /tzdist/zones/America%2FNew_York HTTP/1.1 Host: tz.example.com Accept: application/tzif >> Response << HTTP/1.1 200 OK Date: Fri, 01 Jun 2018 14:52:24 GMT Content-Type: application/tzif Content-Length: xxxx ETag: "123456789-000-111" TZif2...[binary data without leap second records]... EST5EDT,M3.2.0,M11.1.0 ]]>

The Time Zone Information Format contains no executable code and the format does not define any extensible areas that could be used to store such code. TZif contains counted arrays of data elements. All counts should be checked when processing TZif objects to guard against references past the end of the object. TZif provides no confidentiality or integrity protection. Time zone information is normally public and does not call for confidentiality protection. Since time zone information is used in many critical applications, integrity protection may be required, and must be provided externally.

The Time Zone Information Format contains publicly available data and the format does not define any extensible areas that could be used to store private data. As discussed in Section 9 of , transmission of time zone data over an insecure communications channel could leak the past, current, or future location of a device or user. As such, TZif data transmitted over a public communications channel MUST be protected with a confidentiality layer such as that provided by Transport Layer Security (TLS).

This document defines two MIME media types for the exchange of data utilizing the Time Zone Information Format.

application tzif none none binary See of this document. See of this document. This specification. This media type is designed for widespread use by applications that need to use or exchange time zone information, such as the Time Zone Information Compiler (zic) and the GNU C Library. The Time Zone Distribution Service can directly use this media type. N/A The first 4 octets are 0x54, 0x5A, 0x69, 0x66 N/A N/A Time Zone Database mailing list <tz@iana.org> COMMON N/A See the "Author's Address" section of this document. IETF

application tzif-leap none none binary See of this document. See of this document. This specification. This media type is designed for widespread use by applications that need to use or exchange time zone information, such as the Time Zone Information Compiler (zic) and the GNU C Library. The Time Zone Distribution Service can directly use this media type. N/A The first 4 octets are 0x54, 0x5A, 0x69, 0x66 N/A N/A Time Zone Database mailing list <tz@iana.org> COMMON N/A See the "Author's Address" section of this document. IETF

The authors would like to thank the following individuals for contributing their ideas and support for writing this specification: Michael Douglass, Ned Freed, Guy Harris, Eliot Lear, and Alexey Melnikov.

&rfc0020; &rfc2119; &rfc6838; &rfc7231; &rfc7808; &rfc8174; IEEE This standard is also published by ieeexplore.ieee.org. &rfc5545; &rfc6557; &rfc8446;

This section documents common problems in implementing this specification. Most of these are problems in generating TZif files for use by readers conforming to predecessors of this specification. The goals of this section are: to help TZif writers output files that avoid common pitfalls in older or buggy TZif readers, to help TZif readers avoid common pitfalls when reading files generated by future TZif writers, and to help any future specification authors see what sort of problems arise when the TZif format is changed. When new versions of the TZif format have been defined, a design goal has been that a reader can successfully use a TZif file even if the file is of a later TZif version than what the reader was designed for. When complete compatibility was not achieved, an attempt was made to limit glitches to rarely-used timestamps, and to allow simple partial workarounds in writers designed to generate new-version data useful even for older-version readers. This section attempts to document these compatibility issues and workarounds, as well as to document other common bugs in readers. Interoperability problems with TZif include the following: Some readers examine only version 1 data. As a partial workaround, a writer can output as much version 1 data as possible. However, a reader should ignore version 1 data, and should use version 2+ data even if the reader's native timestamps have only 32 bits. Some readers designed for version 2 might mishandle timestamps after a version 3 file's last transition, because they cannot parse extensions to POSIX in the TZ-like string. As a partial workaround, a writer can output more transitions than necessary, so that only far-future timestamps are mishandled by version 2 readers. Some readers designed for version 2 do not support permanent daylight saving time, e.g., a TZ string "EST5EDT,0/0,J365/25" denoting permanent Eastern Daylight Time (-04). As a partial workaround, a writer can substitute standard time for the next time zone east, e.g., "AST4" for permanent Atlantic Standard Time (-04). Some readers ignore the footer, and instead predict future timestamps from the time type of the last transition. As a partial workaround, a writer can output more transitions than necessary. Some readers do not use time type 0 for timestamps before the first transition, in that they infer a time type using a heuristic that does not always select time type 0. As a partial workaround, a writer can output a dummy (no-op) first transition at an early time. Some readers mishandle timestamps before the first transition that has a timestamp not less than -2**31. Readers that support only 32-bit timestamps are likely to be more prone to this problem, for example, when they process 64-bit transitions only some of which are representable in 32 bits. As a partial workaround, a writer can output a dummy transition at timestamp -2**31. Some readers mishandle a transition if its timestamp has the minimum possible signed 64-bit value. Timestamps less than -2**59 are not recommended. Some readers mishandle POSIX-style TZ strings that contain '<' or '>'. As a partial workaround, a writer can avoid using '<' or '>' for time zone abbreviations containing only alphabetic characters. Many readers mishandle time zone abbreviations that contain non-ASCII characters. These characters are not recommended. Some readers may mishandle time zone abbreviations that contain fewer than 3 or more than 6 characters, or that contain ASCII characters other than alphanumerics, '-', and '+'. These abbreviations are not recommended. Some readers mishandle TZif files that specify daylight-saving time UT offsets that are less than the UT offsets for the corresponding standard time. These readers do not support locations like Ireland, which uses the equivalent of the POSIX TZ string "IST-1GMT0,M10.5.0,M3.5.0/1", observing standard time (IST, +01) in summer and daylight saving time (GMT, +00) in winter. As a partial workaround, a writer can output data for the equivalent of the POSIX TZ string "GMT0IST,M3.5.0/1,M10.5.0", thus swapping standard and daylight saving time. Although this workaround misidentifies which part of the year uses daylight saving time, it records UT offsets and time zone abbreviations correctly. Some interoperability problems are reader bugs that are listed here mostly as warnings to developers of readers. Some readers do not support negative timestamps. Developers of distributed applications should keep this in mind if they need to deal with pre-1970 data. Some readers mishandle timestamps before the first transition that has a nonnegative timestamp. Readers that do not support negative timestamps are likely to be more prone to this problem. Some readers mishandle time zone abbreviations like "-08" that contain '+', '-', or digits. Some readers mishandle UT offsets that are out of the traditional range of -12 through +12 hours, and so do not support locations like Kiritimati that are outside this range. Some readers mishandle UT offsets in the range [-3599, -1] seconds from UT, because they integer-divide the offset by 3600 to get 0 and then display the hour part as "+00". Some readers mishandle UT offsets that are not a multiple of one hour, or of 15 minutes, or of 1 minute.

The following sections contain annotated hexadecimal dumps of example TZif files. Note that these examples should only be considered informative. Although the example data entries are current as of the publication date of this document, the data will likely change in the future as leap seconds are added and changes are made to civil time.

File Offset Data Octets (hexadecimal) Record Name / Field Name Field Value 000 54 5a 69 66 magic "TZif" 004 00 version 0 (1) 005 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 020 00 00 00 01 isutccnt 1 024 00 00 00 01 isstdcnt 1 028 00 00 00 1b isleapcnt 27 032 00 00 00 00 timecnt 0 036 00 00 00 01 typecnt 1 040 00 00 00 04 charcnt 4 localtimetype[0] 044 00 00 00 00 utcoff 00:00 048 00 isdst 0 (no) 049 00 desigidx 0 050 55 54 43 00 designations[0] "UTC" leapsecond[0] 054 04 b2 58 00 occurrence 78796800 (1972-06-30T23:59:60Z) 058 00 00 00 01 correction 1 leapsecond[1] 062 05 a4 ec 01 occurrence 94694401 (1972-12-31T23:59:60Z) 066 00 00 00 02 correction 2 leapsecond[2] 070 07 86 1f 82 occurrence 126230402 (1973-12-31T23:59:60Z) 074 00 00 00 03 correction 3 leapsecond[3] 078 09 67 53 03 occurrence 157766403 (1974-12-31T23:59:60Z) 082 00 00 00 04 correction 4 leapsecond[4] 086 0b 48 86 84 occurrence 189302404 (1975-12-31T23:59:60Z) 090 00 00 00 05 correction 5 leapsecond[5] 094 0d 2b 0b 85 occurrence 220924805 (1976-12-31T23:59:60Z) 098 00 00 00 06 correction 6 leapsecond[6] 102 0f 0c 3f 06 occurrence 252460806 (1977-12-31T23:59:60Z) 106 00 00 00 07 correction 7 leapsecond[7] 110 10 ed 72 87 occurrence 283996807 (1978-12-31T23:59:60Z) 114 00 00 00 08 correction 8 leapsecond[8] 118 12 ce a6 08 occurrence 315532808 (1979-12-31T23:59:60Z) 122 00 00 00 09 correction 9 leapsecond[9] 126 15 9f ca 89 occurrence 362793609 (1981-06-30T23:59:60Z) 130 00 00 00 0a correction 10 leapsecond[10] 134 17 80 fe 0a occurrence 394329610 (1982-06-30T23:59:60Z) 138 00 00 00 0b correction 11 leapsecond[11] 142 19 62 31 8b occurrence 425865611 (1983-06-30T23:59:60Z) 146 00 00 00 0c correction 12 leapsecond[12] 150 1d 25 ea 0c occurrence 489024012 (1985-06-30T23:59:60Z) 154 00 00 00 0d correction 13 leapsecond[13] 158 21 da e5 0d occurrence 567993613 (1987-12-31T23:59:60Z) 162 00 00 00 0e correction 14 leapsecond[14] 166 25 9e 9d 8e occurrence 631152014 (1989-12-31T23:59:60Z) 170 00 00 00 0f correction 15 leapsecond[15] 174 27 7f d1 0f occurrence 662688015 (1990-12-31T23:59:60Z) 178 00 00 00 10 correction 16 leapsecond[16] 182 2a 50 f5 90 occurrence 709948816 (1992-06-30T23:59:60Z) 186 00 00 00 11 correction 17 leapsecond[17] 190 2c 32 29 11 occurrence 741484817 (1993-06-30T23:59:60Z) 194 00 00 00 12 correction 18 leapsecond[18] 198 2e 13 5c 92 occurrence 773020818 (1994-06-30T23:59:60Z) 202 00 00 00 13 correction 19 leapsecond[19] 206 30 e7 24 13 occurrence 820454419 (1995-12-31T23:59:60Z) 210 00 00 00 14 correction 20 leapsecond[20] 214 33 b8 48 94 occurrence 867715220 (1997-06-30T23:59:60Z) 218 00 00 00 15 correction 21 leapsecond[21] 222 36 8c 10 15 occurrence 915148821 (1998-12-31T23:59:60Z) 226 00 00 00 16 correction 22 leapsecond[22] 230 43 b7 1b 96 occurrence 1136073622 (2005-12-31T23:59:60Z) 234 00 00 00 17 correction 23 leapsecond[23] 238 49 5c 07 97 occurrence 1230768023 (2008-12-31T23:59:60Z) 242 00 00 00 18 correction 24 leapsecond[24] 246 4f ef 93 18 occurrence 1341100824 (2012-06-30T23:59:60Z) 250 00 00 00 19 correction 25 leapsecond[25] 254 55 93 2d 99 occurrence 1435708825 (2015-06-30T23:59:60Z) 258 00 00 00 1a correction 26 leapsecond[26] 262 58 68 46 9a occurrence 1483228826 (2016-12-31T23:59:60Z) 266 00 00 00 1b correction 27 270 00 UT/local[0] 0 (local) 271 00 standard/wall[0] 0 (wall) To determine TAI corresponding to 2000-01-01T00:00:00Z (UNIX time = 946684800), the following procedure would be followed: Find the the latest leap second occurrence prior to the time of interest (leapsecond[21]) and note the correction value (LEAPCORR = 22). Add LEAPCORR + 10 to the time of interest to yield TAI of 2000-01-01T00:00:32.

File Offset Hexadecimal Octets Record Name / Field Name Field Value 000 54 5a 69 66 magic "TZif" 004 32 version '2' (2) 005 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 020 00 00 00 06 isutccnt 6 024 00 00 00 06 isstdcnt 6 028 00 00 00 00 isleapcnt 0 032 00 00 00 07 timecnt 7 036 00 00 00 06 typecnt 6 040 00 00 00 14 charcnt 20 044 80 00 00 00 trans time[0] -2147483648 (1901-12-13T20:45:52Z) 048 bb 05 43 48 trans time[1] -1157283000 (1933-04-30T12:30:00Z) 052 bb 21 71 58 trans time[2] -1155436200 (1933-05-21T21:30:00Z) 056 cb 89 3d c8 trans time[3] -880198200 (1942-02-09T12:30:00Z) 060 d2 23 f4 70 trans time[4] -769395600 (1945-08-14T23:00:00Z) 064 d2 61 49 38 trans time[5] -765376200 (1945-09-30T11:30:00Z) 068 d5 8d 73 48 trans time[6] -712150200 (1947-06-08T12:30:00Z) 072 01 trans type[0] 1 073 02 trans type[1] 2 074 01 trans type[2] 1 075 03 trans type[3] 3 076 04 trans type[4] 4 077 01 trans type[5] 1 078 05 trans type[6] 5 localtimetype[0] 079 ff ff 6c 02 utcoff -37886 (-10:21:26) 083 00 isdst 0 (no) 084 00 desigidx 0 localtimetype[1] 085 ff ff 6c 58 utcoff -37800 (-10:30) 089 00 isdst 0 (no) 090 04 desigidx 4 localtimetype[2] 091 ff ff 7a 68 utcoff -34200 (-09:30) 095 01 isdst 1 (yes) 096 08 desigidx 8 localtimetype[3] 097 ff ff 7a 68 utcoff -34200 (-09:30) 101 01 isdst 1 (yes) 102 0c desigidx 12 localtimetype[4] 103 ff ff 7a 68 utcoff -34200 (-09:30) 107 01 isdst 1 (yes) 108 10 desigidx 16 localtimetype[5] 109 ff ff 73 60 utcoff -36000 (-10:00) 113 00 isdst 0 (no) 114 04 desigidx 4 115 4c 4d 54 00 designations[0] "LMT" 119 48 53 54 00 designations[4] "HST" 123 48 44 54 00 designations[8] "HDT" 127 48 57 54 00 designations[12] "HWT" 131 48 50 54 00 designations[16] "HPT" 135 00 UT/local[0] 1 (UT) 136 00 UT/local[1] 0 (local) 137 00 UT/local[2] 0 (local) 138 00 UT/local[3] 0 (local) 139 01 UT/local[4] 1 (UT) 140 00 UT/local[5] 0 (local) 141 00 standard/wall[0] 1 (standard) 142 00 standard/wall[1] 0 (wall) 143 00 standard/wall[2] 0 (wall) 144 00 standard/wall[3] 0 (wall) 145 01 standard/wall[4] 1 (standard) 146 00 standard/wall[5] 0 (wall) 147 54 5a 69 66 magic "TZif" 151 32 version '2' (2) 152 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 167 00 00 00 06 isutccnt 6 171 00 00 00 06 isstdcnt 6 175 00 00 00 00 isleapcnt 0 179 00 00 00 07 timecnt 7 183 00 00 00 06 typecnt 6 187 00 00 00 14 charcnt 20 191 ff ff ff ff 74 e0 70 be trans time[0] -2334101314 (1896-01-13T22:31:26Z) 199 ff ff ff ff bb 05 43 48 trans time[1] -1157283000 (1933-04-30T12:30:00Z) 207 ff ff ff ff bb 21 71 58 trans time[2] -1155436200 (1933-05-21T21:30:00Z) 215 ff ff ff ff cb 89 3d c8 trans time[3] -880198200 (1942-02-09T12:30:00Z) 223 ff ff ff ff d2 23 f4 70 trans time[4] -769395600 (1945-08-14T23:00:00Z) 231 ff ff ff ff d2 61 49 38 trans time[5] -765376200 (1945-09-30T11:30:00Z) 239 ff ff ff ff d5 8d 73 48 trans time[6] -712150200 (1947-06-08T12:30:00Z) 247 01 trans type[0] 1 248 02 trans type[1] 2 249 01 trans type[2] 1 250 03 trans type[3] 3 251 04 trans type[4] 4 252 01 trans type[5] 1 253 05 trans type[6] 5 localtimetype[0] 254 ff ff 6c 02 utcoff -37886 (-10:21:26) 258 00 isdst 0 (no) 259 00 desigidx 0 localtimetype[1] 260 ff ff 6c 58 utcoff -37800 (-10:30) 264 00 isdst 0 (no) 265 04 desigidx 4 localtimetype[2] 266 ff ff 7a 68 utcoff -34200 (-09:30) 270 01 isdst 1 (yes) 271 08 desigidx 8 localtimetype[3] 272 ff ff 7a 68 utcoff -34200 (-09:30) 276 01 isdst 1 (yes) 277 0c desigidx 12 localtimetype[4] 278 ff ff 7a 68 utcoff -34200 (-09:30) 282 01 isdst 1 (yes) 283 10 desigidx 16 localtimetype[5] 284 ff ff 73 60 utcoff -36000 (-10:00) 288 00 isdst 0 (no) 289 04 desigidx 4 290 4c 4d 54 00 designations[0] "LMT" 294 48 53 54 00 designations[4] "HST" 298 48 44 54 00 designations[8] "HDT" 302 48 57 54 00 designations[12] "HWT" 306 48 50 54 00 designations[16] "HPT" 310 00 UT/local[0] 0 (local) 311 00 UT/local[1] 0 (local) 312 00 UT/local[2] 0 (local) 313 00 UT/local[3] 0 (local) 314 01 UT/local[4] 1 (UT) 315 00 UT/local[5] 0 (local) 316 00 standard/wall[0] 0 (wall) 317 00 standard/wall[1] 0 (wall) 318 00 standard/wall[2] 0 (wall) 319 00 standard/wall[3] 0 (wall) 320 01 standard/wall[4] 1 (standard) 321 00 standard/wall[5] 0 (wall) 322 0a NL '\n' 323 48 53 54 31 30 TZ string "HST10" 328 0a NL '\n' To determine the local time in this time zone corresponding to 1933-05-04T12:00:00Z (UNIX time = -1156939200), the following procedure would be followed: Find the the latest time transition prior to the time of interest (trans time[1]). Reference the corresponding transition type (trans type[1]) to determine the local time type index (2). Reference the corresponding local time type (localtimetype[2]) to determine the offset from UTC (-09:30), the daylight saving indicator (1 = yes), and the index into the time zone designation strings (8). Lookup the corresponding time zone designation string (designations[8] = "HDT"). Add the UTC offset to the time of interest to yield a local daylight saving time of 1933-05-04T02:30:00-09:30 (HDT). To determine the local time in this time zone corresponding to 2019-01-01T00:00:00Z (UNIX time = 1546300800), the following procedure would be followed: Find the the latest time transition prior to the time of interest (there is no such transition). Lookup the TZ string in the footer ("HST10"), which indicates that the time zone designation is "HST" year round, and the offset to UTC is 10:00. Subtract the UTC offset from the time of interest to yield a standard local time of 2018-12-31T14:00:00-10:00 (HST).

The following TZif file has been truncated to start on 2038-01-01T00:00:00Z. File Offset Hexadecimal Octets Record Name / Field Name Field Value 000 54 5a 69 66 magic "TZif" 004 33 version '3' (3) 005 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 020 00 00 00 00 isutccnt 0 024 00 00 00 00 isstdcnt 0 028 00 00 00 00 isleapcnt 0 032 00 00 00 00 timecnt 0 036 00 00 00 00 typecnt 0 040 00 00 00 00 charcnt 0 044 54 5a 69 66 magic "TZif" 048 33 version '3' (3) 049 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 064 00 00 00 03 isutccnt 1 068 00 00 00 03 isstdcnt 1 072 00 00 00 00 isleapcnt 0 076 00 00 00 03 timecnt 1 080 00 00 00 03 typecnt 1 084 00 00 00 08 charcnt 4 088 00 00 00 00 7f e8 17 80 trans time[0] 2145916800 (2038-01-01T00:00:00Z) 096 00 trans type[0] 0 localtimetype[0] 097 00 00 1c 20 utcoff 7200 (+02:00) 101 00 isdst 0 (no) 102 00 desigidx 0 103 49 53 54 00 designations[0] "IST" 107 01 UT/local[0] 1 (UT) 108 01 standard/wall[0] 1 (standard) 109 0a NL '\n' 110 49 53 54 2d 32 49 44 54 2c 4d 33 2e 34 2e 34 2f 32 36 2c 4d 31 30 2e 35 2e 30 TZ string "IST-2IDT, M3.4.4/26,M10.5.0" 136 0a NL '\n'

Changes since -15: Addressed IESG comments from Ben Campbell. Addressed IESG comments from Spencer Dawkins. Addressed IESG comments from Benjamin Kaduk. Added annotated example files. Minor editorial changes. Changes since -14: Addressed last call comments from Tom Petch. Addressed last call comments from Qin Wu. Addressed last call comments from Dale Worley. Changes since -13: Added text to Privacy Considerations. Changes since -12: Added reference to RFC0020. Fully fleshed out application/tzif-leap declaration rather than referencing application/tzif. Added definition for "Leap Second Correction". Added external references directly to the relevant sections of POSIX. Changes since -11: Removed text requiring leapcnt by non-zero for application/tzif-leap files. Changes since -10: Removed text mandating all TZDIST features be supported. Minor editorial changes. Changes since -09: Removed "Update 7808" from header as this spec doesn't introduce new TZDIST functionality. Added text regarding truncation of TZif via TZDIST. Expanded what this spec DOESN'T define. Added reasons for some of the recommended practices. Added common interoperability issues appendix. Minor editorial changes. Changes since -08: Clarifying text regarding MIME types. Consolidated/referenced duplicate security and interoperability text. Switched to 'octets' instead of 'characters' when describing time zone designations. Minor editorial changes. Changes since -07: Clarifying text regarding TZ string. Added "application/tzif-leap" MIME media type. New reference for zic(8) man page. Minor editorial changes. Changes since -06: Added definition of UNIX Leap Time and used it to describe transition times and leap second occurrences. Moved TZif generation recommendations into discussion of version field. Repeated TZif generation recommendations in TZDIST section. Rewrote part of the TZ string text. Minor editorial changes. Changes since -05: Clarify TAI, leap seconds, some descriptions, and some field values/ranges with text from Paul Eggert. Refined MIME declaration based on feedback from Ned Freed. Changes since -04: Edited text discussing timestamps before first and after last transition. Specified legal range of time type indices and time zone designation indices. Notes that corrections in adjacent leap second records must differ by one. Added recommendations to eliminate unused space. Minor editorial changes. Changes since -03: Removed definition of GMT. Updated definitions of UTC, TAI, and UT Switched to using UT rather than UTC. Added more text about the use of standard/wall and UT/local indicators. Added Acknowledgments. Minor editorial changes. Changes since -02: Updated definitions of Standard Time and DST. Added definitions of GMT and UT. Added a definition of Time Zone Data from RFC7808. Removed sentence stating that TZDB is accurate. Added more text for standard/wall and UTC/local indicators and counts. Added text discussing timestamps before first and after last transition. Added more guidance text regarding 32-bit and 64-bit data consistency. Minor editorial changes. Changes since -01: Renamed "POSIX Time" to "UNIX Time" and noted that values can be negative. Noted that signed values MUST be represented using two's complement. Renamed "POSIX TZ string" to "TZ string" and noted that it can be empty. Moved TZ string extensions into its own subsection with examples. Renamed leap second "epoch" to "occurrence". Editorial changes from Paul Eggert. Changes since -00: Split TZif format description into a general overview and 3 subsections. Updated Keywords boilerplate. Updated POSIX reference. Editorial changes from Eliot Lear.