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Checking references for intended status: Proposed Standard ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) No issues found here. Summary: 0 errors (**), 0 flaws (~~), 1 warning (==), 4 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group C. Daboo 3 Internet-Draft Apple Inc. 4 Updates: 5545, 4791 (if approved) G. Yakushev 5 Intended status: Standards Track Google Inc. 6 Expires: December 13, 2014 June 11, 2014 8 Non-Gregorian Recurrence Rules in iCalendar 9 draft-daboo-icalendar-rscale-04 11 Abstract 13 This document defines how non-Gregorian recurrence rules can be 14 specified in iCalendar data. 16 Status of this Memo 18 This Internet-Draft is submitted in full conformance with the 19 provisions of BCP 78 and BCP 79. 21 Internet-Drafts are working documents of the Internet Engineering 22 Task Force (IETF). Note that other groups may also distribute 23 working documents as Internet-Drafts. The list of current Internet- 24 Drafts is at http://datatracker.ietf.org/drafts/current/. 26 Internet-Drafts are draft documents valid for a maximum of six months 27 and may be updated, replaced, or obsoleted by other documents at any 28 time. It is inappropriate to use Internet-Drafts as reference 29 material or to cite them other than as "work in progress." 31 This Internet-Draft will expire on December 13, 2014. 33 Copyright Notice 35 Copyright (c) 2014 IETF Trust and the persons identified as the 36 document authors. All rights reserved. 38 This document is subject to BCP 78 and the IETF Trust's Legal 39 Provisions Relating to IETF Documents 40 (http://trustee.ietf.org/license-info) in effect on the date of 41 publication of this document. Please review these documents 42 carefully, as they describe your rights and restrictions with respect 43 to this document. Code Components extracted from this document must 44 include Simplified BSD License text as described in Section 4.e of 45 the Trust Legal Provisions and are provided without warranty as 46 described in the Simplified BSD License. 48 Table of Contents 50 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 51 2. Conventions Used in This Document . . . . . . . . . . . . . . 4 52 3. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 53 4. Extended RRULE Property . . . . . . . . . . . . . . . . . . . 6 54 4.1. Handling Leap Months . . . . . . . . . . . . . . . . . . . 7 55 4.2. Examples . . . . . . . . . . . . . . . . . . . . . . . . . 7 56 5. Registering Calendar Systems . . . . . . . . . . . . . . . . . 10 57 6. Use with iTIP . . . . . . . . . . . . . . . . . . . . . . . . 11 58 7. Use with CalDAV . . . . . . . . . . . . . . . . . . . . . . . 11 59 7.1. CALDAV:supported-rscale-set Property . . . . . . . . . . . 12 60 8. Security Considerations . . . . . . . . . . . . . . . . . . . 13 61 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13 62 10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 13 63 11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13 64 11.1. Normative References . . . . . . . . . . . . . . . . . . . 13 65 11.2. Informative References . . . . . . . . . . . . . . . . . . 14 66 Appendix A. Change History (To be removed by RFC Editor 67 before publication) . . . . . . . . . . . . . . . . . 14 68 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 15 70 1. Introduction 72 The iCalendar [RFC5545] data format is in widespread use to represent 73 calendar data. iCalendar represents dates and times using the 74 Gregorian calendar system only. It does provide a way to use non- 75 Gregorian calendar systems via a "CALSCALE" property, however this 76 has never been formally used. However, there is a need to support at 77 least non-Gregorian recurrence patterns to cover anniversaries, and 78 many local, religious, or civil holidays based on non-Gregorian 79 dates. 81 There are several disadvantages to using the existing "CALSCALE" 82 property in iCalendar for implementing non-Gregorian calendars: 84 1. The "CALSCALE" property exists in the top-level "VCALENDAR" 85 objects and thus applies to all components within that object. 86 In today's multi-cultural society, that restricts the ability to 87 mix events from different calendar systems within the same 88 iCalendar object. e.g., it would prevent having both the 89 Gregorian New Year and Chinese New Year in the same iCalendar 90 object. 92 2. Many countries observe daylight savings time, encoded in 93 iCalendar using the "VTIMEZONE" component. Timezone and daylight 94 saving time rules are always specified via Gregorian calendar 95 based recurrence rules (e.g., "the 3rd Sunday in March"). This 96 is problematic for non-Gregorian uses of "CALSCALE" which would 97 by default also apply to the dates and rules used in the 98 "VTIMEZONE" components in the corresponding iCalendar object. 100 This specification solves these issues by allowing the "CALSCALE" to 101 remain set to Gregorian, but re-defining the recurrence rule property 102 "RRULE" to accept new items including one that allows non-Gregorian 103 calendar systems to be used. With this, all the date, time and 104 period values in the iCalendar object would remain specified using 105 the Gregorian calendar system, but repeating patterns in other 106 calendar systems could be defined. It is then up to calendar user 107 agents and servers to map between Gregorian and non-Gregorian 108 calendar systems in order to expand out recurrence instances. 110 This specification does not itself define calendar systems, rather it 111 utilizes the calendar system registry defined by the Unicode 112 Consortium in their CLDR (Common Locale Data Repository) project 113 [UNICODE.CLDR]. 115 2. Conventions Used in This Document 117 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 118 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 119 "OPTIONAL" in this document are to be interpreted as described in 120 [RFC2119]. 122 The notation used in this memo is the ABNF notation of [RFC5234] as 123 used by iCalendar [RFC5545]. Any syntax elements shown below that 124 are not explicitly defined in this specification come from iCalendar 125 [RFC5545], iTIP [RFC5546], and CalDAV [RFC4791]. 127 When XML element types in the namespaces "DAV:" and 128 "urn:ietf:params:xml:ns:caldav" are referenced in this document 129 outside of the context of an XML fragment, the string "DAV:" and 130 "CALDAV:" will be prefixed to the element type names respectively. 132 When a Gregorian calendar date value is shown in text, it will use 133 the format "YYYYMMHH", where "YYYY" is the 4-digit year, "MM" the 134 2-digit month, and "DD" the 2-digit day (this is the same format used 135 in iCalendar [RFC5545]). The Chinese calendar will be used as an 136 example of a non-Gregorian calendar for illustrative purposes. When 137 a Chinese calendar date value is shown in text, it will use the 138 format "{C}YYYYMM[L]DD" - i.e., the same format as Gregorian but with 139 a "{C}" prefix, and an optional "L" character after the month element 140 to indicate a leap month. Similarly, {E} and {H} are used in other 141 examples as prefixes for Ethiopic (Amete Mihret) and Hebrew dates, 142 respectively. Note that the Chinese calendar years shown in the 143 examples are based on the Unicode (ICU) [UNICODE.ICU] library's 144 Chinese calendar epoch. Whilst there are several different Chinese 145 calendar epochs in common use, the choice of one over another does 146 not impact the actual calculation of the Gregorian equivalent dates, 147 provided conversion is always done using the same epoch. 149 3. Overview 151 In the Gregorian calendar system, each year is composed of a fixed 152 number of months (12), with each month having a fixed number of days 153 (between 30 and 31), except for the second month (February) which 154 contains either 28 days, or 29 days (in a leap year). Weeks are 155 composed of 7 days, with day names Monday, Tuesday, Wednesday, 156 Thursday, Friday, Saturday and Sunday. Years can have either 365 or 157 366 days (the later in a leap year). The number of whole weeks in a 158 year is 52. 160 In iCalendar, the "RECUR" value type defines various fields used to 161 express a recurrence pattern, and those fields are given limits based 162 on those of the Gregorian calendar system. Since other calendar 163 systems can have different limits and other behaviors that need to be 164 accounted for, the maximum values for the elements in the "RECUR" 165 value are not covered by this specification. 167 To generate a set of recurring instances in a non-Gregorian calendar 168 system, the following procedure is used: 170 1. iCalendar data continues to use the "GREGORIAN" calendar system, 171 so all "DATE", "DATE-TIME" and "PERIOD" values continue to use 172 the Gregorian format and limits. 174 2. The "RRULE" property is extended to include an "RSCALE" element 175 in its value that specifies the calendar system to use for the 176 recurrence pattern. The existing elements of the "RRULE" value 177 type are used, but modified to support different upper limits, 178 based on the "RSCALE" value, as well as a modification to month 179 numbers to allow a leap month to be specified. Existing 180 requirements for the use of "RRULE" all still apply (e.g., the 181 "RRULE" has to match the "DTSTART" value of the master instance). 182 Other recurrence properties such as "RECURRENCE-ID", "RDATE" and 183 "EXDATE" continue to use the Gregorian date format as "CALSCALE" 184 is unchanged. 186 When generating instances, the following procedure might be used: 188 1. Convert the "DTSTART" property value of the master recurring 189 component into the date and time components for the calendar 190 system specified by the "RSCALE" element in the "RRULE" value. 191 This provides the "seed" value for generating subsequent 192 recurrence instances. 194 2. Iteratively generate instances using the "RRULE" value applied to 195 the year, month, and day components of the date in the new 196 calendar system. 198 3. For each generated instance, convert the date values back from 199 the non-Gregorian form into Gregorian and use those values for 200 other properties such as "RECURRENCE-ID". 202 Consider the following example for an event representing the Chinese 203 New Year: 205 DTSTART;VALUE=DATE:20130210 206 RRULE:RSCALE=CHINESE;FREQ=YEARLY 207 SUMMARY:Chinese New Year 209 To generate instances, first the "DTSTART" value "20130210" is 210 converted into the Chinese calendar system giving "{C}46500101". 211 Next, the year component is incremented by one to give "{C}46510101", 212 and that is then converted back into Gregorian as "20140131". 213 Additional instances are generated by iteratively increasing the year 214 component in the Chinese date value and converting back to Gregorian. 216 4. Extended RRULE Property 218 This specification extends the existing "RRULE" iCalendar property 219 value to include a new "RSCALE" element that can be used to indicate 220 the calendar system used for generating the recurrence pattern. 222 When "RSCALE" is present, the other changes to "RRULE" are: 224 1. Elements that include numeric values (e.g., "BYYEARDAY") have 225 numeric ranges defined by the "RSCALE" value (i.e., in some 226 calendar systems there might be more than 366 days in a year). 228 2. Month numbers can include an "L" suffix to indicate that the 229 specified month is a leap month in the corresponding calendar 230 system. 232 3. A "SKIP" element is added to define how "missing" instances are 233 handled. e.g., if a yearly recurring event starts in a leap 234 month, the "SKIP" element determines whether instances in non- 235 leap years are ignored ("SKIP" set to "YES"), appear in the 236 preceding regular month ("SKIP" set to "BACKWARD" - the default 237 when "RSCALE" is present), or appear in the following regular 238 month ("SKIP" set to "FORWARD"). This applies for both leap days 239 and leap months. The "SKIP" processing is done after all rule 240 elements, other than "BYSETPOS", "COUNT" and "UNTIL", have been 241 processed. 243 The syntax for the "RECUR" value is modified in the following 244 fashion: 246 recur-rule-part /= ("RSCALE" "=" rscale) 247 / ("SKIP" "=" skip) 249 rscale = (iana-token ; A CLDR-registered calendar system 250 ; name. 251 / x-name) ; A non-standard, experimental 252 ; calendar system name. 253 ; Names are case-insensitive, 254 ; but uppercase values are preferred. 256 skip = ("YES" / "BACKWARD" / "FORWARD") 257 ; Optional, with default value "BACKWARD", 258 ; and MUST only be present if "RSCALE" is present. 260 monthnum = 1*2DIGIT ["L"] 261 ; Existing element modified to include a leap 262 ; month indicator suffix. 264 4.1. Handling Leap Months 266 Leap months can occur in different calendar systems. For such 267 calendar systems the following rules are applied for "identifying" 268 months: 270 1. Numeric values 1 through N are used to identify regular, non- 271 leap, months (where N is the number of months in a regular, non- 272 leap, year). 274 2. The suffix "L" is added to the regular month number to indicate a 275 leap month which follows the regular month. e.g., "5L" is a leap 276 month that follows the 5th regular month in the year. 278 Care has to be taken when mapping the month identifiers used here 279 with those of any underlying calendar system library being used. In 280 particular, the Hebrew calendar system used by Unicode (ICU) 281 [UNICODE.ICU] uses a month number scheme of 1 through 13, with month 282 6 being the leap month, and in non-leap years, month 6 is skipped. 283 In iCalendar, this would map to months 1 through 12 with "5L" as the 284 leap month. 286 4.2. Examples 288 4.2.1. Chinese New Year 290 Consider the following set of iCalendar properties: 292 DTSTART;VALUE=DATE:20130210 293 RRULE:RSCALE=CHINESE;FREQ=YEARLY 294 SUMMARY:Chinese New Year 296 These define a recurring event for the Chinese New Year, with the 297 first instance the one in Gregorian year 2013. 299 The Chinese date corresponding to the first instance is {C}46500101. 300 The table below shows the initial instance, and the next four, each 301 of which is determined by adding the appropriate amount to the year 302 component of the Chinese date. Also shown is the conversion back to 303 the Gregorian date: 305 +--------------+--------------------------+ 306 | Chinese Date | Gregorian Date | 307 +--------------+--------------------------+ 308 | {C}46500101 | 20130210 - DTSTART value | 309 | {C}46510101 | 20140131 | 310 | {C}46520101 | 20150219 | 311 | {C}46530101 | 20160208 | 312 | {C}46540101 | 20170128 | 313 +--------------+--------------------------+ 315 4.2.2. Ethiopic 13th Month 317 Consider the following set of iCalendar properties: 319 DTSTART;VALUE=DATE:201300906 320 RRULE:RSCALE=ETHIOPIC;FREQ=YEARLY;BYMONTH=13 321 SUMMARY:First day of 13th month 323 These define a recurring event for the first day of the 13th month, 324 with the first instance the one in Gregorian year 2013. 326 The Ethiopic date corresponding to the first instance is {E}20051301. 327 The table below shows the initial instance, and the next four, each 328 of which is determined by adding the appropriate amount to the year 329 component of the Ethiopic date. Also shown is the conversion back to 330 the Gregorian date: 332 +---------------+--------------------------+ 333 | Ethiopic Date | Gregorian Date | 334 +---------------+--------------------------+ 335 | {E}20051301 | 20130906 - DTSTART value | 336 | {E}20061301 | 20140906 | 337 | {E}20071301 | 20150906 | 338 | {E}20081301 | 20160906 | 339 | {E}20091301 | 20170906 | 340 +---------------+--------------------------+ 342 Note that in this example, the value of the "BYMONTH" component in 343 the "RRULE" matches the Ethiopic month value and not the Gregorian 344 month. 346 4.2.3. Hebrew anniversary starting in a leap month 348 Consider the following set of iCalendar properties: 350 DTSTART;VALUE=DATE:20140208 351 RRULE:RSCALE=HEBREW;FREQ=YEARLY;BYMONTH=5L;BYMONTHDAY=8;SKIP=FORWARD 352 SUMMARY:Anniversary 354 These define a recurring event for the 8th day of the Hebrew month of 355 Adar I (the leap month identified by "5L"), with the first instance 356 the one in Gregorian year 2014. 358 The Hebrew date corresponding to the first instance is {H}577405L08, 359 which is a leap month in year 5774. The table below shows the 360 initial instance, and the next four, each of which is determined by 361 adding the appropriate amount to the year component of the Hebrew 362 date, taking into account that only year 5776 is a leap year. Thus 363 in other years the Hebrew month component is adjusted forward to 364 month 6. Also shown is the conversion back to the Gregorian date: 366 +--------------+--------------------------+ 367 | Hebrew Date | Gregorian Date | 368 +--------------+--------------------------+ 369 | {H}577405L08 | 20140208 - DTSTART value | 370 | {H}57750608 | 20150227 | 371 | {H}577605L08 | 20160217 | 372 | {H}57770608 | 20170306 | 373 | {H}57780608 | 20180223 | 374 +--------------+--------------------------+ 376 4.2.4. Gregorian leap day with SKIP 378 Consider the following set of iCalendar properties: 380 DTSTART;VALUE=DATE:20120229 381 RRULE:FREQ=YEARLY 382 SUMMARY:Anniversary 384 These define a recurring event for the 29th February, 2012 in the 385 standard iCalendar calendar scale - Gregorian. The standard 386 iCalendar behavior is that non-existent dates in a recurrence set are 387 ignored. Thus the properties above would only generate instances in 388 leap years (2016, 2020, etc), which is likely not what users expect. 389 The new "RSCALE" option defined by this specification provides the 390 "SKIP" element which can be used to "fill in" the missing instances 391 in an appropriate fashion. The set of iCalendar properties below do 392 that: 394 DTSTART;VALUE=DATE:20120229 395 RRULE:RSCALE=GREGORIAN;FREQ=YEARLY;SKIP=FORWARD 396 SUMMARY:Anniversary 398 With these properties, the "missing" instances in non-leap year now 399 appear on the 1st March in those years: 401 +-------------------------------+----------------------------+ 402 | Instances (with SKIP=FORWARD) | Instances (without RSCALE) | 403 +-------------------------------+----------------------------+ 404 | 20120229 | 20120229 - DTSTART value | 405 | 20130301 | | 406 | 20140301 | | 407 | 20150301 | | 408 | 20160229 | 20160229 | 409 | 20170301 | | 410 +-------------------------------+----------------------------+ 412 5. Registering Calendar Systems 414 This specification uses the Unicode Consortium's registry of calendar 415 systems [UNICODE.CLDR] to define valid values for the "RSCALE" 416 element of an "RRULE". Note that the underscore character "_" is 417 never used in CLDR-based calendar system names. New values can be 418 added to this registry following Unicode Consortium rules. It is 419 expected that many implementations of non-Gregorian calendars will 420 use software libraries provided by Unicode (ICU) [UNICODE.ICU], and 421 hence it makes sense to re-use their registry rather than creating a 422 new one. For consistency, when used, the "RSCALE" values SHOULD be 423 uppercased. 425 CLDR supports the use of "alias" values as alternative names for 426 specific calendar systems. These alias values MUST be treated as 427 valid "RSCALE" element values. 429 When using the CLDR data, calendar agents SHOULD take into account 430 the "deprecated" value and use the alternative "preferred" calendar 431 system. In particular, the "islamicc" calendar system is considered 432 deprecated in favor of the "islamic-civil" calendar system. 434 6. Use with iTIP 436 iTIP [RFC5546] defines how iCalendar data can be sent between 437 calendar user agents to schedule calendar components between calendar 438 users. It is often not possible to know the capabilities of a 439 calendar user agent to which an iTIP message is being sent, but iTIP 440 defines fallback behavior in such cases. 442 For calendar user agents that do not support the "RSCALE" element, 443 the following can occur when iTIP messages containing an "RSCALE" 444 element are received: 446 The receiving calendar user agent can reject the entire iTIP 447 message and return an iTIP reply with a "REQUEST-STATUS" property 448 set to the "3.1" status code (as per Section 3.6.14 of [RFC5546]). 450 The receiving calendar user agent can fallback to a non-recurring 451 behavior for the calendar component (effectively ignoring the 452 "RRULE" property) and return an iTIP reply with a "REQUEST-STATUS" 453 property set to the "2.3", "2.5", "2.8", or "2.10" status codes 454 (as per Sections 3.6.3, 3.6.6, 3.6.9, or 3.6.11, respectively, of 455 [RFC5546]). 457 For calendar user agents that support the "RSCALE" element but do not 458 support the calendar system specified by the "RSCALE" element value, 459 the following can occur: 461 the iTIP message SHOULD be rejected, returning a "REQUEST-STATUS" 462 property set to the "3.1" status code (as per Section 3.6.14 of 463 [RFC5546]). 465 if the iTIP message is accepted and the calendar component treated 466 as non-recurring, an iTIP reply with a "REQUEST-STATUS" property 467 set to the "2.8" or "2.10" status codes (as per Sections 3.6.9 or 468 3.6.11, respectively, of [RFC5546]) SHOULD be returned. 470 7. Use with CalDAV 472 The CalDAV [RFC4791] calendar access protocol allows clients and 473 server to exchange iCalendar data. In addition, CalDAV clients are 474 able to query calendar data stored on the server, including time- 475 based queries. Since an "RSCALE" element value determines the time 476 ranges for recurring instances in a calendar component, CalDAV 477 servers need to support it to interoperate with clients also using 478 the "RSCALE" element. 480 A CalDAV server advertises a CALDAV:supported-rscale-set WebDAV 481 property on calendar home or calendar collections if it supports use 482 of "RSCALE" element as described in this specification. The server 483 can advertise a specific set of supported calendar systems by 484 including one or more CALDAV:supported-rscale XML elements within the 485 CALDAV:supported-rscale-set XML element. If no CALDAV:supported- 486 rscale XML elements are included in the WebDAV property, then clients 487 can try any calendar system value, but need to be prepared for a 488 failure when attempting to store the calendar data. 490 Clients MUST NOT attempt to store iCalendar data containing "RSCALE" 491 elements if the CALDAV:supported-rscale-set WebDAV property is not 492 advertised by the server. 494 The server SHOULD return an HTTP 403 response with a DAV:error 495 element containing a CALDAV:supported-rscale XML element, if a client 496 attempts to store iCalendar data with an "RSCALE" element value not 497 supported by the server. 499 It is possible for a "RSCALE" value to be present in calendar data on 500 the server being accessed by a client that does not support an 501 "RSCALE" element or its specified value. It is expected that 502 existing clients, unaware of "RSCALE", will fail gracefully by 503 ignoring the calendar component, whilst still processing other 504 calendar data on the server. 506 7.1. CALDAV:supported-rscale-set Property 508 Name: supported-rscale-set 510 Namespace: urn:ietf:params:xml:ns:caldav 512 Purpose: Enumerates the set of supported iCalendar "RSCALE" element 513 values supported by the server. 515 Protected: This property MUST be protected and SHOULD NOT be 516 returned by a PROPFIND allprop request (as defined in Section 14.2 517 of [RFC4918]). 519 Description: See above. 521 Definition: 523 524 525 528 Example: 530 532 GREGORIAN 533 CHINESE 534 ISLAMIC-CIVIL 535 HEBREW 536 ETHIOPIC 537 539 8. Security Considerations 541 This specification does not introduce any addition security concerns 542 beyond those described in [RFC5545], [RFC5546], and [RFC4791]. 544 9. IANA Considerations 546 This specification does not define any new IANA registries or values. 548 10. Acknowledgments 550 Thanks to the following for feedback: Mark Davis, Mike Douglass, 551 Peter Edberg, Marten Gajda, Arnaud Quillaud, Dave Thewlis, and Umaoka 552 Yoshito. This specification came about via discussions at the 553 Calendaring and Scheduling Consortium. 555 11. References 557 11.1. Normative References 559 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 560 Requirement Levels", BCP 14, RFC 2119, March 1997. 562 [RFC4791] Daboo, C., Desruisseaux, B., and L. Dusseault, 563 "Calendaring Extensions to WebDAV (CalDAV)", RFC 4791, 564 March 2007. 566 [RFC4918] Dusseault, L., "HTTP Extensions for Web Distributed 567 Authoring and Versioning (WebDAV)", RFC 4918, June 2007. 569 [RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax 570 Specifications: ABNF", STD 68, RFC 5234, January 2008. 572 [RFC5545] Desruisseaux, B., "Internet Calendaring and Scheduling 573 Core Object Specification (iCalendar)", RFC 5545, 574 September 2009. 576 [RFC5546] Daboo, C., "iCalendar Transport-Independent 577 Interoperability Protocol (iTIP)", RFC 5546, 578 December 2009. 580 [UNICODE.CLDR] 581 "CLDR calendar.xml Data", Unicode Consortium CLDR, 582 August 2013, . 585 11.2. Informative References 587 [UNICODE.ICU] 588 "International Components for Unicode", Unicode 589 Consortium ICU, April 2014, . 591 Appendix A. Change History (To be removed by RFC Editor before 592 publication) 594 Changes in -04: 596 1. Always use "L" suffix for leap months, even for Hebrew calendar. 598 2. Remove negative month numbers to go back to base 5545 definition. 600 3. Added example for Gregorian leap day with skip. 602 4. Clarify that RSCALE names are case insensitive, but with upper 603 case preferred. 605 5. Clarify that BYSETPOS processing is done after SKIP. 607 6. Remove Islamic example in favor of Ethiopic example which shows a 608 13th month. 610 Changes in -03: 612 1. Added details about handling RSCALE in iTIP. 614 2. Added details about handling RSCALE in CalDAV. 616 3. Fixed examples to use ICU Chinese epoch and added text describing 617 why that is not an issue for actual recurrence calculations. 619 Changes in -02: 621 1. Fixed some incorrect dates in examples. 623 2. Clarified use of CLDR and alias, deprecated, preferred 624 attributes. 626 3. Clarified when SKIP processing occurs. 628 Changes in -01: 630 1. Removed requirement that RSCALE be the first item in an RRULE. 632 2. Added BYLEAPMONTH element and removed BYMONTH "L" suffix. 634 3. Removed Open Issues. 636 Authors' Addresses 638 Cyrus Daboo 639 Apple Inc. 640 1 Infinite Loop 641 Cupertino, CA 95014 642 USA 644 Email: cyrus@daboo.name 645 URI: http://www.apple.com/ 647 Gregory Yakushev 648 Google Inc. 649 Brandschenkestrasse 100 650 8002 Zurich, 651 Switzerland 653 Email: yakushev@google.com 654 URI: http://www.google.com/