Network Working Group                                           C. Daboo
Internet-Draft                                                Apple Inc.
Updates: 4791, 5545, 5546, 6321, 7265                        G. Yakushev (if approved)                                       Google Inc.                      G. Yakushev
Intended status: Standards Track                       December 10, 2014                             Google Inc.
Expires: June 13, July 11, 2015                                   January 7, 2015

              Non-Gregorian Recurrence Rules in iCalendar


   This document defines how extensions to iCalendar (RFC 5545) to support
   use of non-Gregorian recurrence rules rules.  It also defines how CalDAV
   (RFC 4791) servers and clients can be
   specified and used in iCalendar data, and with CalDAV servers. extended to support these new
   recurrence rules.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on June 13, July 11, 2015.

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   document authors.  All rights reserved.

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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions Used in This Document . . . . . . . . . . . . . .   3
   3.  Overview  . . . . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  Extended RRULE Property . . . . . . . . . . . . . . . . . . .   5   6
     4.1.  Handling Leap Months  . . . . . . . . . . . . . . . . . .   6   7
     4.2.  Examples  . . . . . . . . . . . . . . . . . . . . . . . .   7
   5.  Registering Calendar Systems  . . . . . . . . . . . . . . . .   9  10
   6.  Use with iTIP . . . . . . . . . . . . . . . . . . . . . . . .  10
   7.  Use with xCal . . . . . . . . . . . . . . . . . . . . . . . .  11
   8.  Use with jCal . . . . . . . . . . . . . . . . . . . . . . . .  11  12
   9.  Use with CalDAV . . . . . . . . . . . . . . . . . . . . . . .  12  13
     9.1.  CALDAV:supported-rscale-set Property  . . . . . . . . . .  13
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  14
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  14
   12. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  14
   13. References  . . . . . . . . . . . . . . . . . . . . . . . . .  14
     13.1.  Normative References . . . . . . . . . . . . . . . . . .  14
     13.2.  Informative References . . . . . . . . . . . . . . . . .  15
   Appendix A.  Change History (To be removed by RFC Editor before
                publication) . . . . . . . . . . . . . . . . . . . .  15
   Appendix B.  xCal RELAX NG schema update  . . . . . . . . . . . .  16  17
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  17  18

1.  Introduction

   The iCalendar [RFC5545] data format is in widespread use to represent
   calendar data. iCalendar represents dates and times using the
   Gregorian calendar system only.  It does provide a way to use non-
   Gregorian calendar systems via a "CALSCALE" property, but this has
   never been used.  However, there is a need to support at least non-
   Gregorian recurrence patterns to cover anniversaries, and many local,
   religious, or civil holidays based on non-Gregorian dates.

   There are several disadvantages to using the existing "CALSCALE"
   property in iCalendar for implementing non-Gregorian calendars:

   1.  The "CALSCALE" property exists in the top-level "VCALENDAR"
       objects and thus applies to all components within that object.
       In today's multi-cultural society, that restricts the ability to
       mix events from different calendar systems within the same
       iCalendar object. e.g., it would prevent having both the
       Gregorian New Year and Chinese New Year in the same iCalendar

   2.  Many countries observe daylight savings time, encoded in
       iCalendar using the "VTIMEZONE" component.  Time zone and daylight saving time rules are always specified via typically published
       using Gregorian calendar based recurrence dates and rules (e.g., "the 3rd Sunday
       March"). March"), and thus converted to iCalendar "VTIMEZONE"
       components using Gregorian date-time values and recurrence rules.
       This results in the problem whereby one component (the
       "VTIMEZONE") is problematic for non-Gregorian uses of
       "CALSCALE" which would by default also apply fixed to the dates Gregorian calendar system, and
       rules used in the "VTIMEZONE" components in
       another (a "VEVENT") wants to use a different non-Gregorian
       calendar scale, and thus the corresponding
       iCalendar object. single top-level "CALSCALE" property
       is again inadequate.

   This specification solves these issues by allowing the "CALSCALE" to
   remain set to Gregorian, but re-defining the "RRULE" recurrence rule
   "RRULE" to accept new items including one that allows non-Gregorian
   calendar systems to be used.  With this, all the date, time and
   period values in the iCalendar object would remain specified using
   the Gregorian calendar system, but repeating patterns in other
   calendar systems could be defined.  It is then up to calendar user
   agents and servers to map between Gregorian and non-Gregorian
   calendar systems in order to expand out recurrence instances.  The
   non-Gregorian recurrence rules can be used in any iCalendar component
   that allows the "RRULE" property to be specified, including
   "VTIMEZONE" components (to allow for possible future use of non-
   Gregorian rules in published daylight saving time data).

   This specification does not itself define calendar systems, rather it
   utilizes the calendar system registry defined by the Unicode
   Consortium in their CLDR (Common Locale Data Repository) project
   [UNICODE.CLDR], as implemented in the Unicode (ICU) Library

   This specification makes the following updates:

      It updates iCalendar [RFC5545], xCal [RFC6321], and jCal
      [RFC7265], to extend the "RRULE" property definition.

      It updates iTIP [RFC5546] to specify how the extended "RRULE"
      property should be handled in iTIP messages.

      It updates CalDAV [RFC4791] to specify how the extended "RRULE"
      property can be supported by CalDAV servers and clients.

2.  Conventions Used in This Document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in

   The notation used in this memo is the ABNF notation of [RFC5234] as
   used by iCalendar [RFC5545].  Any syntax elements shown below that
   are not explicitly defined in this specification come from iCalendar
   [RFC5545], iTIP [RFC5546], and CalDAV [RFC4791].

   When XML element types in the namespaces "DAV:" and
   "urn:ietf:params:xml:ns:caldav" are referenced in this document
   outside of the context of an XML fragment, the string "DAV:" and
   "CALDAV:" will be prefixed to the element type names respectively.

   When a Gregorian calendar date value is shown in text, it will use
   the format "YYYYMMDD", where "YYYY" is the 4-digit year, "MM" the
   2-digit month, and "DD" the 2-digit day (this is the same format used
   in iCalendar [RFC5545]).  The Chinese calendar will be used as an
   example of a non-Gregorian calendar for illustrative purposes.  When
   a Chinese calendar date value is shown in text, it will use the
   format "{C}YYYYMM[L]DD" - i.e., the same format as Gregorian but with
   a "{C}" prefix, and an optional "L" character after the month element
   to indicate a leap month.  Similarly, {E} and {H} are used in other
   examples as prefixes for Ethiopic (Amete Mihret) and Hebrew dates,
   respectively.  Note that the Chinese calendar years shown in the
   examples are based on the Unicode (ICU) [UNICODE.ICU] library's
   Chinese calendar epoch.  Whilst there are several different Chinese
   calendar epochs in common use, the choice of one over another does
   not impact the actual calculation of the Gregorian equivalent dates,
   provided conversion is always done using the same epoch.

3.  Overview

   In the Gregorian calendar system, each year is composed of a fixed
   number of months (12), with each month having a fixed number of days
   (between 30 and 31), except for the second month (February) which
   contains either 28 days, or 29 days (in a leap year).  Weeks are
   composed of 7 days, with day names Monday, Tuesday, Wednesday,
   Thursday, Friday, Saturday and Sunday.  Years can have either 365 or
   366 days (the latter in a leap year).  The number of whole weeks in a
   year is 52 (though the [ISO.8601.2004] week numbering scheme used by
   iCalendar [RFC5545] can have a numeric count up to 53).

   In iCalendar, the "RECUR" value type defines various fields used to
   express a recurrence pattern, and those fields are given limits based
   on those of the Gregorian calendar system.  Since other calendar
   systems can have different limits and other behaviors that need to be
   accounted for, the maximum values for the elements in the "RECUR"
   value are not covered by this specification.

   To generate a set of recurring instances in a non-Gregorian calendar
   system, the following principles are used:

   1.  iCalendar data continues to use the "GREGORIAN" calendar system,
       so all "DATE", "DATE-TIME" and "PERIOD" values continue to use
       the Gregorian format and limits.

   2.  The "RRULE" property is extended to include an "RSCALE" element
       in its value that specifies the calendar system to use for the
       recurrence pattern.  The existing elements of the "RRULE" value
       type are used, but modified to support different upper limits,
       based on the "RSCALE" value, as well as a modification to month
       numbers to allow a leap month to be specified.  Existing
       requirements for the use of "RRULE" all still apply (e.g., the
       "RRULE" has to match the "DTSTART" value of the master instance).
       Other recurrence properties such as "RECURRENCE-ID", "RDATE" and
       "EXDATE" continue to use the Gregorian date format as "CALSCALE"
       is unchanged.

   When generating instances, the following procedure might be used:

   1.  Convert the "DTSTART" property value of the master recurring
       component into the date and time components for the calendar
       system specified by the "RSCALE" element in the "RRULE" value.
       This provides the "seed" value for generating subsequent
       recurrence instances.

   2.  Iteratively generate instances using the "RRULE" value applied to
       the year, month, and day components of the date in the new
       calendar system.

   3.  For each generated instance, convert the date values back from
       the non-Gregorian form into Gregorian and use those values for
       other properties such as "RECURRENCE-ID".

   Consider the following example for an event representing the Chinese
   New Year:

   SUMMARY:Chinese New Year

   To generate instances, first the "DTSTART" value "20130210" is
   converted into the Chinese calendar system giving "{C}46500101".
   Next, the year component is incremented by one to give "{C}46510101",
   and that is then converted back into Gregorian as "20140131".
   Additional instances are generated by iteratively increasing the year
   component in the Chinese date value and converting back to Gregorian.

4.  Extended RRULE Property

   This specification extends the existing "RRULE" iCalendar property
   value to include a new "RSCALE" element that can be used to indicate
   the calendar system used for generating the recurrence pattern.

   When "RSCALE" is present, the other changes to "RRULE" are:

   1.  Elements that include numeric values (e.g., "BYYEARDAY") have
       numeric ranges defined by the "RSCALE" value (i.e., in some
       calendar systems there might be more than 366 days in a year).

   2.  Month numbers can include an "L" suffix to indicate that the
       specified month is a leap month in the corresponding calendar

   3.  A "SKIP" element is added to define how "missing" instances are
       handled. e.g., if a yearly recurring event starts in a leap
       month, the "SKIP" element determines whether instances in non-
       leap years are ignored ("SKIP" set to "YES"), appear in the
       preceding regular month ("SKIP" set to "BACKWARD" - the default
       when "RSCALE" is present), or appear in the following regular
       month ("SKIP" set to "FORWARD").  This applies for both leap days
       and leap months.  The "SKIP" processing is done after all rule
       elements, other than "BYSETPOS", "COUNT" and "UNTIL", have been

   The syntax for the "RECUR" value is modified in the following

   recur-rule-part /=   ("RSCALE" "=" rscale)
                      / ("SKIP" "=" skip)

   rscale          = (iana-token  ; A CLDR-registered calendar system
                                  ; name.
                    / x-name)     ; A non-standard, experimental
                                  ; calendar system name.
                                  ; Names are case-insensitive,
                                  ; but uppercase values are preferred.

   skip            = ("YES" / "BACKWARD" / "FORWARD")
                    ; Optional, with default value "BACKWARD",
                    ; and MUST only be present if "RSCALE" is present.

   monthnum        = 1*2DIGIT  ["L"]
                    ; Existing element modified to include a leap
                    ; month indicator suffix.

4.1.  Handling Leap Months

   Leap months can occur in different calendar systems.  For such
   calendar systems the following rules are applied for "identifying"

   1.  Numeric values 1 through N are used to identify regular, non-
       leap, months (where N is the number of months in a regular, non-
       leap, year).

   2.  The suffix "L" is added to the regular month number to indicate a
       leap month which follows the regular month. e.g., "5L" is a leap
       month that follows the 5th regular month in the year.

   Care has to be taken when mapping the month identifiers used here
   with those of any underlying calendar system library being used.  In
   particular, the Hebrew calendar system used by Unicode (ICU)
   [UNICODE.ICU] uses a month number scheme of 1 through 13, with month
   6 being the leap month, and in non-leap years, month 6 is skipped.
   In iCalendar, this would map to months 1 through 12 with "5L" as the
   leap month.

4.2.  Examples

4.2.1.  Chinese New Year

   Consider the following set of iCalendar properties:

   SUMMARY:Chinese New Year

   These define a recurring event for the Chinese New Year, with the
   first instance the one in Gregorian year 2013.

   The Chinese date corresponding to the first instance is {C}46500101.
   The table below shows the initial instance, and the next four, each
   of which is determined by adding the appropriate amount to the year
   component of the Chinese date.  Also shown is the conversion back to
   the Gregorian date:

                | Chinese Date | Gregorian Date           |
                | {C}46500101  | 20130210 - DTSTART value |
                | {C}46510101  | 20140131                 |
                | {C}46520101  | 20150219                 |
                | {C}46530101  | 20160208                 |
                | {C}46540101  | 20170128                 |

4.2.2.  Ethiopic 13th Month

   Consider the following set of iCalendar properties:

   SUMMARY:First day of 13th month

   These define a recurring event for the first day of the 13th month,
   with the first instance the one in Gregorian year 2013.

   The Ethiopic date corresponding to the first instance is {E}20051301.
   The table below shows the initial instance, and the next four, each
   of which is determined by adding the appropriate amount to the year
   component of the Ethiopic date.  Also shown is the conversion back to
   the Gregorian date:

               | Ethiopic Date | Gregorian Date           |
               | {E}20051301   | 20130906 - DTSTART value |
               | {E}20061301   | 20140906                 |
               | {E}20071301   | 20150906                 |
               | {E}20081301   | 20160906                 |
               | {E}20091301   | 20170906                 |

   Note that in this example, the value of the "BYMONTH" component in
   the "RRULE" matches the Ethiopic month value and not the Gregorian

4.2.3.  Hebrew anniversary starting in a leap month

   Consider the following set of iCalendar properties:

   These define a recurring event for the 8th day of the Hebrew month of
   Adar I (the leap month identified by "5L"), with the first instance
   the one in Gregorian year 2014.

   The Hebrew date corresponding to the first instance is {H}577405L08,
   which is a leap month in year 5774.  The table below shows the
   initial instance, and the next four, each of which is determined by
   adding the appropriate amount to the year component of the Hebrew
   date, taking into account that only year 5776 is a leap year.  Thus
   in other years the Hebrew month component is adjusted forward to
   month 6.  Also shown is the conversion back to the Gregorian date:

                | Hebrew Date  | Gregorian Date           |
                | {H}577405L08 | 20140208 - DTSTART value |
                | {H}57750608  | 20150227                 |
                | {H}577605L08 | 20160217                 |
                | {H}57770608  | 20170306                 |
                | {H}57780608  | 20180223                 |

4.2.4.  Gregorian leap day with SKIP

   Consider the following set of iCalendar properties:


   These define a recurring event for the 29th February, 2012 in the
   standard iCalendar calendar scale - Gregorian.  The standard
   iCalendar behavior is that non-existent dates in a recurrence set are
   ignored.  Thus the properties above would only generate instances in
   leap years (2016, 2020, etc), which is likely not what users expect.
   The new "RSCALE" option defined by this specification provides the
   "SKIP" element which can be used to "fill in" the missing instances
   in an appropriate fashion.  The set of iCalendar properties below do


   With these properties, the "missing" instances in non-leap year now
   appear on the 1st March in those years:

      | Instances (with SKIP=FORWARD) | Instances (without RSCALE) |
      | 20120229                      | 20120229 - DTSTART value   |
      | 20130301                      |                            |
      | 20140301                      |                            |
      | 20150301                      |                            |
      | 20160229                      | 20160229                   |
      | 20170301                      |                            |

5.  Registering Calendar Systems

   This specification uses the Unicode Consortium's registry of calendar
   systems [UNICODE.CLDR] to define valid values for the "RSCALE"
   element of an "RRULE".  Note that the underscore character "_" is
   never used in CLDR-based calendar system names.  New values can be
   added to this registry following Unicode Consortium rules.  It is
   expected that many implementations of non-Gregorian calendars will
   use software libraries provided by Unicode (ICU) [UNICODE.ICU], and
   hence it makes sense to re-use their registry rather than creating a
   new one.  For consistency, when used, the "RSCALE" values SHOULD be

   CLDR supports the use of "alias" values as alternative names for
   specific calendar systems.  These alias values MUST be treated as
   valid "RSCALE" element values.

   When using the CLDR data, calendar agents SHOULD take into account
   the "deprecated" value and use the alternative "preferred" calendar
   system.  In particular, the "islamicc" calendar system is considered
   deprecated in favor of the "islamic-civil" calendar system.

6.  Use with iTIP

   iTIP [RFC5546] defines how iCalendar data can be sent between
   calendar user agents to schedule calendar components between calendar
   users.  It is often not possible to know the capabilities of a
   calendar user agent to which an iTIP message is being sent, but iTIP
   defines fallback behavior in such cases.

   For calendar user agents that do not support the "RSCALE" element,
   the following can occur when iTIP messages containing an "RSCALE"
   element are received:

      The receiving calendar user agent can reject the entire iTIP
      message and return an iTIP reply with a "REQUEST-STATUS" property
      set to the "3.1" status code (as per Section 3.6.14 of [RFC5546]).

      The receiving calendar user agent can fallback to a non-recurring
      behavior for the calendar component (effectively ignoring the
      "RRULE" property) and return an iTIP reply with a "REQUEST-STATUS"
      property set to the "2.3", "2.5", "2.8", or "2.10" status codes
      (as per Sections 3.6.3, 3.6.6, 3.6.9, or 3.6.11, respectively, of

   For calendar user agents that support the "RSCALE" element but do not
   support the calendar system specified by the "RSCALE" element value,
   the following can occur:

      the iTIP message SHOULD be rejected, returning a "REQUEST-STATUS"
      property set to the "3.1" status code (as per Section 3.6.14 of

      if the iTIP message is accepted and the calendar component treated
      as non-recurring, an iTIP reply with a "REQUEST-STATUS" property
      set to the "2.8" or "2.10" status codes (as per Sections 3.6.9 or
      3.6.11, respectively, of [RFC5546]) SHOULD be returned.

7.  Use with xCal

   xCal [RFC6321] defines how iCalendar data is represented in XML.
   This specification extends the <recur> XML element in Section 3.6.10
   of [RFC6321] in the following manner:

   1.  A new <rscale> XML element is defined as a child element of
       <recur>.  The content of this element MUST be a string whose
       value is the "RSCALE" element value of the "RRULE", with case

   2.  A new <skip> XML element is defined as a child element of
       <recur>.  The content of this element MUST be a string whose
       value is the "SKIP" element value of the "RRULE", with case

   3.  The "bymonth" XML element is redefined to support either numeric
       or string values as its content (as per Section 4.1).

   Extensions to the RELAX NG schema in Appendix A of [RFC6321] are
   defined in Appendix B.

   Example: the iCalendar "RRULE" property:


   would be represented in XML as:


8.  Use with jCal

   jCal [RFC7265] defines how iCalendar data is represented in JSON.
   This specification extends the "recur" JSON object defined in
   Section 3.6.10 of [RFC7265] in the following manner:

   1.  A new "rscale" child member is defined.  This MUST be a string
       whose value is the "RSCALE" element value of the "RRULE", with
       case preserved.

   2.  A new "skip" child member is defined.  This MUST be a string
       whose value is the "SKIP" element value of the "RRULE", with case

   3.  The "bymonth" child member is redefined to support either numeric
       or string values.  If the "BYMONTH" element value is an integer,
       then a numeric JSON value MUST be used.  If the "BYMONTH" element
       value is an integer with the "L" suffix (as per Section 4.1),
       then a JSON string value MUST be used.

   Example: the iCalendar "RRULE" property:


   would be represented in JSON as:

       "rscale": "GREGORIAN",
       "freq": "YEARLY",
       "skip": "FORWARD"

9.  Use with CalDAV

   The CalDAV [RFC4791] calendar access protocol allows clients and
   servers to exchange iCalendar data.  In addition, CalDAV clients are
   able to query calendar data stored on the server, including time-
   based queries.  Since an "RSCALE" element value determines the time
   ranges for recurring instances in a calendar component, CalDAV
   servers need to support it to interoperate with clients also using
   the "RSCALE" element.

   A CalDAV server advertises a CALDAV:supported-rscale-set WebDAV
   property on calendar home or calendar collections if it supports use
   of "RSCALE" element as described in this specification.  The server
   can advertise a specific set of supported calendar systems by
   including one or more CALDAV:supported-rscale XML elements within the
   CALDAV:supported-rscale-set XML element.  If no CALDAV:supported-
   rscale XML elements are included in the WebDAV property, then clients
   can try any calendar system value, but need to be prepared for a
   failure when attempting to store the calendar data.

   Clients MUST NOT attempt to store iCalendar data containing "RSCALE"
   elements if the CALDAV:supported-rscale-set WebDAV property is not
   advertised by the server.

   The server SHOULD return an HTTP 403 response with a DAV:error
   element containing a CALDAV:supported-rscale XML element, if a client
   attempts to store iCalendar data with an "RSCALE" element value not
   supported by the server.

   It is possible for an "RSCALE" value to be present in calendar data
   on the server being accessed by a client that does not support an
   "RSCALE" element or its specified value.  It is expected that
   existing clients, unaware of "RSCALE", will fail gracefully by
   ignoring the calendar component, whilst still processing other
   calendar data on the server.

9.1.  CALDAV:supported-rscale-set Property

   Name:  supported-rscale-set

   Namespace:  urn:ietf:params:xml:ns:caldav

   Purpose:  Enumerates the set of supported iCalendar "RSCALE" element
      values supported by the server.

   Protected:  This property MUST be protected and SHOULD NOT be
      returned by a PROPFIND allprop request (as defined in Section 14.2
      of [RFC4918]).

   Description:  See above.


   <!ELEMENT supported-rscale-set (supported-rscale*) >
   <!ELEMENT supported-rscale (#PCDATA)>
   <!-- PCDATA value: string - case-insensitive but
        uppercase preferred -->



10.  Security Considerations

   This specification does not introduce any addition security concerns
   beyond those described in [RFC5545], [RFC5546], and [RFC4791].

11.  IANA Considerations

   This document requires no IANA actions.

12.  Acknowledgments

   Thanks to the following for feedback: Mark Davis, Mike Douglass,
   Donald Eastlake, Peter Edberg, Marten Gajda, Philipp Kewisch,
   Jonathan Lennox, Ken Murchison, Arnaud Quillaud, Dave Thewlis, and
   Umaoka Yoshito.

   This specification originated from work at the Calendaring and
   Scheduling Consortium, which has helped with the development and
   testing of implementations.

13.  References

13.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC4791]  Daboo, C., Desruisseaux, B., and L. Dusseault,
              "Calendaring Extensions to WebDAV (CalDAV)", RFC 4791,
              March 2007.

   [RFC4918]  Dusseault, L., "HTTP Extensions for Web Distributed
              Authoring and Versioning (WebDAV)", RFC 4918, June 2007.

   [RFC5234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234, January 2008.

   [RFC5545]  Desruisseaux, B., "Internet Calendaring and Scheduling
              Core Object Specification (iCalendar)", RFC 5545,
              September 2009.

   [RFC5546]  Daboo, C., "iCalendar Transport-Independent
              Interoperability Protocol (iTIP)", RFC 5546, December

   [RFC6321]  Daboo, C., Douglass, M., and S. Lees, "xCal: The XML
              Format for iCalendar", RFC 6321, August 2011.

   [RFC7265]  Kewisch, P., Daboo, C., and M. Douglass, "jCal: The JSON
              Format for iCalendar", RFC 7265, May 2014.

              "CLDR calendar.xml Data", Unicode Consortium CLDR,

13.2.  Informative References

              International Organization for Standardization, "Data
              elements and interchange formats - Information interchange
              - Representation of dates and times", ISO Standard 8601,

              "International Components for Unicode", Unicode Consortium
              ICU, April 2014, <>.

Appendix A.  Change History (To be removed by RFC Editor before

   Changes in draft-ietf-calext-rscale-03:

   1.  Reworded abstract.

   2.  Added list of changes to other specs in Section 1.

   3.  Clarified behavior wrt VTIMEZONE in Section 1.

   Changes in draft-ietf-calext-rscale-02:

   1.  Added xCal and jCal changes sections and relax NG schema

   2.  Added ICU reference at the end of Section 1.

   Changes in draft-ietf-calext-rscale-01:

   1.  Editorial changes/fixes per document shepherd review.

   2.  Switched CLDR reference to "tags/latest".

   Changes in draft-ietf-calext-rscale-00:

   1.  Updated some references.

   2.  Editorial changes/fixes.

   Changes in draft-daboo-icalendar-rscale-04:

   1.  Always use "L" suffix for leap months, even for Hebrew calendar.

   2.  Remove negative month numbers to go back to base 5545 definition.

   3.  Added example for Gregorian leap day with skip.

   4.  Clarify that RSCALE names are case insensitive, but with upper
       case preferred.

   5.  Clarify that BYSETPOS processing is done after SKIP.

   6.  Remove Islamic example in favor of Ethiopic example which shows a
       13th month.

   Changes in draft-daboo-icalendar-rscale-03:

   1.  Added details about handling RSCALE in iTIP.

   2.  Added details about handling RSCALE in CalDAV.

   3.  Fixed examples to use ICU Chinese epoch and added text describing
       why that is not an issue for actual recurrence calculations.

   Changes in draft-daboo-icalendar-rscale-02:

   1.  Fixed some incorrect dates in examples.

   2.  Clarified use of CLDR and alias, deprecated, preferred

   3.  Clarified when SKIP processing occurs.

   Changes in draft-daboo-icalendar-rscale-01:

   1.  Removed requirement that RSCALE be the first item in an RRULE.

   2.  Added BYLEAPMONTH element and removed BYMONTH "L" suffix.

   3.  Removed Open Issues.

Appendix B.  xCal RELAX NG schema update

   The following changes are made to the RELAX NG schema defined in
   Appendix A of [RFC6321].

   # 3.3.10 RECUR
   # This extension adds type-rscale and type-skip,
   # and modifies type-bymonth

   value-recur = element recur {
       (type-until | type-count)?,
       element interval {
       element wkst { type-weekday }?,

   type-rscale = element rscale {

   type-bymonth = element bymonth {
       xsd:positiveInteger |

   type-skip = element skip {
       "YES" |
       "BACKWARD" |

Authors' Addresses
   Cyrus Daboo
   Apple Inc.
   1 Infinite Loop
   Cupertino, CA  95014


   Gregory Yakushev
   Google Inc.
   Brandschenkestrasse 100
   8002 Zurich