Non-Gregorian Recurrence Rules in the Internet Calendaring and Scheduling Core Object Specification (iCalendar)
RFC 7529

Internet Engineering Task Force (IETF)                          C. Daboo
Request for Comments: 7529                                    Apple Inc.
Updates: 5545, 6321, 7265                                    G. Yakushev
Category: Standards Track                                    Google Inc.
ISSN: 2070-1721                                                 May 2015

     Non-Gregorian Recurrence Rules in the Internet Calendaring and
            Scheduling Core Object Specification (iCalendar)

Abstract

   This document defines extensions to the Internet Calendaring and
   Scheduling Core Object Specification (iCalendar) (RFC 5545) to
   support use of non-Gregorian recurrence rules.  It also defines how
   Calendaring Extensions to WebDAV (CalDAV) (RFC 4791) servers and
   clients can be extended to support these new recurrence rules.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc7529.

Copyright Notice

   Copyright (c) 2015 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions Used in This Document . . . . . . . . . . . . . .   3
   3.  Overview  . . . . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  Extended RRULE Property . . . . . . . . . . . . . . . . . . .   6
     4.1.  Skipping Invalid Dates  . . . . . . . . . . . . . . . . .   6
     4.2.  Handling Leap Months  . . . . . . . . . . . . . . . . . .   9
     4.3.  Examples  . . . . . . . . . . . . . . . . . . . . . . . .  10
   5.  Registering Calendar Systems  . . . . . . . . . . . . . . . .  13
   6.  Compatibility . . . . . . . . . . . . . . . . . . . . . . . .  13
   7.  Use with iTIP . . . . . . . . . . . . . . . . . . . . . . . .  14
   8.  Use with xCal . . . . . . . . . . . . . . . . . . . . . . . .  15
   9.  Use with jCal . . . . . . . . . . . . . . . . . . . . . . . .  15
   10. Use with CalDAV . . . . . . . . . . . . . . . . . . . . . . .  16
     10.1.  CALDAV:supported-rscale-set Property . . . . . . . . . .  17
   11. Security Considerations . . . . . . . . . . . . . . . . . . .  18
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  18
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  18
     12.2.  Informative References . . . . . . . . . . . . . . . . .  19
   Appendix A.  xCal RELAX NG Schema Update  . . . . . . . . . . . .  20
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  21
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  21

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
       object.

   2.  Time zone and daylight saving time rules are typically published
       using Gregorian calendar dates and rules (e.g., "the 3rd Sunday
       in March") and are thus converted to iCalendar "VTIMEZONE"

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       components using Gregorian dates and recurrence rules.  This
       results in the problem whereby one component (the "VTIMEZONE") is
       fixed to the Gregorian calendar system, and another (a "VEVENT")
       wants to use a different non-Gregorian calendar scale; thus, the
       single top-level "CALSCALE" property is again inadequate.

   This specification solves these issues by allowing the "CALSCALE" to
   remain set to Gregorian but redefining the "RRULE" recurrence rule
   property to accept new items, including one that allows non-Gregorian
   calendar systems to be used.  With this, all the "DATE", "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 Common Locale Data Repository (CLDR) project
   [UNICODE.CLDR], as implemented in the Unicode (International
   Components for Unicode (ICU)) Library [UNICODE.ICU].

   This specification makes the following updates:

      It updates iCalendar [RFC5545], the XML format for iCalendar
      (xCal) [RFC6321], and the JSON format for iCalendar (jCal)
      [RFC7265], to extend the "RRULE" property definition.

      It clarifies use of the iCalendar Transport-Independent
      Interoperability Protocol (iTIP) [RFC5546] to specify how the
      extended "RRULE" property should be handled in iTIP messages.

      It extends 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",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   [RFC2119].

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   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 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 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.  The "{}" prefix is used for purely illustrative
   purposes and never appears in actual dates used in iCalendar or
   related specifications.  Note that the Chinese calendar years shown
   in the examples are based on the Unicode (ICU) [UNICODE.ICU]
   library's Chinese calendar epoch.  While 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 or 29 days (the latter 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.

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   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 dates and times 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:

   DTSTART;VALUE=DATE:20130210
   RRULE:RSCALE=CHINESE;FREQ=YEARLY
   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 and converting back to Gregorian.

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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
       system (see Section 4.2).

   3.  A "SKIP" element is added to define how "missing" instances are
       handled (see Section 4.1).

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

   ; recur-rule-part is extended from RFC 5545
   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            = ("OMIT" / "BACKWARD" / "FORWARD")
                    ; Optional, with default value "OMIT", and
                    ; MUST NOT be present unless "RSCALE" is present.

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

4.1.  Skipping Invalid Dates

   In every calendar system, only certain combinations of day-of-month
   and month values are valid for a given year, e.g., in the Gregorian
   calendar system, January 31st is valid but February 31st is not.
   Similarly, February 29th is valid in a leap year but invalid in a
   non-leap year.  Other calendar systems can also include leap months

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   (see Section 4.2), which vary from year to year.  This poses a
   problem for recurring events where the frequency of recurrence might
   give rise to an invalid date.  For example, a recurring event that
   starts on January 31st and is set to repeat monthly will generate
   invalid dates for months with fewer than 31 days.  The iCalendar
   [RFC5545] specification requires recurrence rule generators to ignore
   any invalid dates generated when iterating the rule.  However, that
   behavior might be surprising to a calendar user born on a leap day
   and whose birthday event only appears on their calendar every four
   years.  There are common conventions used by humans to determine what
   to do in such cases, but those conventions can differ from calendar
   system to calendar system, as well as within the same calendar
   system, depending on the nature of the event.  Typically, humans will
   expect the "missing" events to be moved to an earlier or later
   (valid) date.

   This specification introduces a new "RRULE" element, "SKIP", for use
   only when the "RSCALE" element is present.  The "SKIP" element allows
   the calendar user agent to specify new options for handling invalid
   dates.

      "SKIP=OMIT": this is the default option and corresponds to the
      existing iCalendar behavior of simply ignoring the invalid date.

      "SKIP=BACKWARD": when this option is set, a date with an invalid
      month is changed to the previous (valid) month.  A date with an
      invalid day-of-month is changed to the previous (valid)
      day-of-month.

      "SKIP=FORWARD": when this option is set, a date with an invalid
      month is changed to the next (valid) month.  A date with an
      invalid day-of-month is changed to the next (valid) day-of-month.

   Note that for both "BACKWARD" and "FORWARD", if the month is changed
   and results in an invalid day-of-month, then the skip behavior will
   be reapplied as per the day-of-month rules, according to the
   processing order defined below.

   The month and day-of-month skip behavior is only applied at specific
   points during the processing of an "RRULE" as determined by the order
   in which any "BYxxx" elements are processed.  The order is as follows
   (based on the "RRULE" element processing order defined in
   Section 3.3.10 of [RFC5545]):

   o  BYMONTH

   o  SKIP (for invalid month only)

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   o  BYWEEKNO

   o  BYYEARDAY

   o  BYMONTHDAY

   o  SKIP (for invalid day)

   o  BYDAY

   o  BYHOUR

   o  BYMINUTE

   o  BYSECOND

   o  BYSETPOS

   o  COUNT

   o  UNTIL

   It is often possible to avoid having to deal with invalid dates by
   determining the real intent of a human user, e.g., a human creating a
   monthly recurring event that starts on January 31st likely intends
   the event to occur on the last day of every month, in which case that
   could be encoded into an "RRULE" by using the "BYMONTHDAY=-1"
   element.

   Only a few types of recurrence patterns are likely to need the use of
   "SKIP".  The following is a list of some situations where it might be
   needed:

   1.  The start date of the recurrence is a leap day in the specified
       calendar system.

   2.  The start date of the recurrence is in a leap month in the
       specified calendar system.

   3.  The start date of the recurrence has a day-of-month value greater
       than the smallest day-of-month value for any month in any year in
       the specified calendar system.

   4.  A "BYMONTHDAY" element in an "RRULE" has a day-of-month value
       greater than the smallest day-of-month value for any month in any
       year in the specified calendar system.

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   5.  A "BYMONTH" element in an "RRULE" has a value corresponding to a
       leap month in the specified calendar system.

   6.  A combination of a "BYMONTHDAY" element and a "BYMONTH" element
       in an "RRULE" has a value corresponding to a leap day in the
       specified calendar system.

   7.  A "BYYEARDAY" element in an "RRULE" has an absolute value greater
       than the smallest number of days in any year in the specified
       calendar system.

   8.  A "BYWEEKNO" element in an "RRULE" has an absolute value greater
       than the smallest number of weeks in any year in the specified
       calendar system.

   Examples of using "SKIP" for some common use cases appear in
   Section 4.3.

4.2.  Handling Leap Months

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

   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 that 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.
   Thus, ICU months 1 through 5 map to iCalendar months 1 through 5, ICU
   month 6 maps to iCalendar month "5L", and ICU months 7 through 13 map
   to iCalendar months 6 through 12.

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4.3.  Examples

4.3.1.  Chinese New Year

   Consider the following set of iCalendar properties (from the example
   above):

   DTSTART;VALUE=DATE:20130210
   RRULE:RSCALE=CHINESE;FREQ=YEARLY
   SUMMARY:Chinese New Year

   These define a recurring event for the Chinese New Year, with the
   first instance being 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                 |
                +--------------+--------------------------+

4.3.2.  Ethiopic 13th Month

   Consider the following set of iCalendar properties:

   DTSTART;VALUE=DATE:20130906
   RRULE:RSCALE=ETHIOPIC;FREQ=MONTHLY;BYMONTH=13
   SUMMARY:First day of 13th month

   These define a recurring event for the first day of the 13th month,
   with the first instance being the one in Gregorian year 2013.  While
   there are a number of alternative ways of writing the "RRULE" above
   to achieve the same pattern of recurring dates, the one above was
   chosen to illustrate a "BYMONTH" value exceeding the limit of 12,
   previously described in iCalendar (Section 3.3.10 of [RFC5545]).

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   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
   month.

4.3.3.  Hebrew Anniversary Starting in a Leap Month

   Consider the following set of iCalendar properties:

   DTSTART;VALUE=DATE:20140208
   RRULE:RSCALE=HEBREW;FREQ=YEARLY;BYMONTH=5L;BYMONTHDAY=8;SKIP=FORWARD
   SUMMARY:Anniversary

   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
   being 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:

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

4.3.4.  Gregorian Leap Day with SKIP

   Consider the following set of iCalendar properties:

   DTSTART;VALUE=DATE:20120229
   RRULE:FREQ=YEARLY
   SUMMARY:Anniversary

   These define a recurring event for the 29th of 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
   does that:

   DTSTART;VALUE=DATE:20120229
   RRULE:RSCALE=GREGORIAN;FREQ=YEARLY;SKIP=FORWARD
   SUMMARY:Anniversary

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

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

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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 reuse their registry rather than creating a
   new one.  "RSCALE" values are case insensitive, but uppercase is
   preferred.

   CLDR supports the use of "alias" values as alternative names for
   specific calendar systems.  These alias values can be used as
   "RSCALE" values and are treated the same as the equivalent CLDR
   calendar system they are an alias for.

   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.  Compatibility

   For calendar user agents that do not support the "RSCALE" element, or
   do not support the calendar system specified by the "RSCALE" element
   value, the following behaviors are possible when processing iCalendar
   data:

   1.  The calendar user agent can reject the entire iCalendar object
       within which at least one iCalendar component uses the
       unrecognized "RSCALE" element or element value.

   2.  The calendar user agent can reject just the iCalendar components
       containing an unrecognized "RSCALE" element or element value.
       Note that any overridden components associated with those
       rejected components MUST also be rejected (i.e., any other
       components with the same "UID" property value as the one with the
       unrecognized "RSCALE" element or element value).

   3.  The calendar user agent can fall back to a non-recurring behavior
       for the iCalendar component containing the unrecognized "RSCALE"
       element or element value (effectively ignoring the "RRULE"
       property).  However, any overridden components SHOULD be rejected
       as they would represent "orphaned" instances that would seem to
       be out of place.

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   In general, the best choice for a calendar user agent would be option
   (2) above, as it would be the least disruptive choice.  Note that
   when processing iTIP [RFC5546] messages, the manner of the rejection
   is covered as discussed in the next section.

7.  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 fall back 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 3.6.4, 3.6.6, 3.6.9, or 3.6.11, respectively, of
      [RFC5546]).

   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
      [RFC5546]).

      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.

   As noted in Section 6, the best choice is to reject the entire iTIP
   message.

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8.  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
       preserved.

   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
       preserved.

   3.  The <bymonth> XML element is redefined to support either numeric
       or string values as its content (as per Section 4.2).

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

   Example: the iCalendar "RRULE" property:

   RRULE:RSCALE=GREGORIAN;FREQ=YEARLY;SKIP=FORWARD

   would be represented in XML as:

   <rrule>
     <recur>
       <rscale>GREGORIAN</rscale>
       <freq>YEARLY</freq>
       <skip>FORWARD</skip>
     </recur>
   </rrule>

9.  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.

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   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
       preserved.

   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 4.2),
       then a JSON string value MUST be used.

   Example: the iCalendar "RRULE" property:

   RRULE:RSCALE=GREGORIAN;FREQ=YEARLY;SKIP=FORWARD

   would be represented in JSON as:

   [
     "rrule",
     {},
     "recur",
     {
       "rscale": "GREGORIAN",
       "freq": "YEARLY",
       "skip": "FORWARD"
     }
   ]

10.  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 Web
   Distributed Authoring and Versioning (WebDAV) property on calendar
   home or calendar collections if it supports use of the "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.

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   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, while still processing other
   calendar data on the server (as per option (2) in Section 6).

10.1.  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.

   Definition:

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

   Example:

   <C:supported-rscale-set
        xmlns:C="urn:ietf:params:xml:ns:caldav">
     <C:supported-rscale>GREGORIAN</C:supported-rscale>
     <C:supported-rscale>CHINESE</C:supported-rscale>
     <C:supported-rscale>ISLAMIC-CIVIL</C:supported-rscale>
     <C:supported-rscale>HEBREW</C:supported-rscale>
     <C:supported-rscale>ETHIOPIC</C:supported-rscale>
   </C:supported-rscale-set>

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11.  Security Considerations

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

12.  References

12.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC4791]  Daboo, C., Desruisseaux, B., and L. Dusseault,
              "Calendaring Extensions to WebDAV (CalDAV)", RFC 4791,
              March 2007, <http://www.rfc-editor.org/info/rfc4791>.

   [RFC4918]  Dusseault, L., Ed., "HTTP Extensions for Web Distributed
              Authoring and Versioning (WebDAV)", RFC 4918, June 2007,
              <http://www.rfc-editor.org/info/rfc4918>.

   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234, January 2008,
              <http://www.rfc-editor.org/info/rfc5234>.

   [RFC5545]  Desruisseaux, B., Ed., "Internet Calendaring and
              Scheduling Core Object Specification (iCalendar)", RFC
              5545, September 2009,
              <http://www.rfc-editor.org/info/rfc5545>.

   [RFC5546]  Daboo, C., Ed., "iCalendar Transport-Independent
              Interoperability Protocol (iTIP)", RFC 5546, December
              2009, <http://www.rfc-editor.org/info/rfc5546>.

   [RFC6321]  Daboo, C., Douglass, M., and S. Lees, "xCal: The XML
              Format for iCalendar", RFC 6321, August 2011,
              <http://www.rfc-editor.org/info/rfc6321>.

   [RFC7265]  Kewisch, P., Daboo, C., and M. Douglass, "jCal: The JSON
              Format for iCalendar", RFC 7265, May 2014,
              <http://www.rfc-editor.org/info/rfc7265>.

   [UNICODE.CLDR]
              The Unicode Consortium, "CLDR calendar.xml Data", Unicode
              Consortium CLDR,
              <http://www.unicode.org/repos/cldr/tags/latest/common/
              bcp47/calendar.xml>.

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12.2.  Informative References

   [ISO.8601.2004]
              International Organization for Standardization, "Data
              elements and interchange formats - Information interchange
              - Representation of dates and times", ISO Standard 8601,
              December 2004.

   [UNICODE.ICU]
              "International Components for Unicode", April 2014,
              <http://site.icu-project.org>.

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Appendix A.  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-rscale?,
       type-freq,
       (type-until | type-count)?,
       element interval {
           xsd:positiveInteger
       }?,
       type-bysecond*,
       type-byminute*,
       type-byhour*,
       type-byday*,
       type-bymonthday*,
       type-byyearday*,
       type-byweekno*,
       type-bymonth*,
       type-bysetpos*,
       element wkst { type-weekday }?,
       type-skip?
   }

   type-rscale = element rscale {
       xsd:string
   }

   type-bymonth = element bymonth {
       xsd:positiveInteger |
       xsd:string
   }

   type-skip = element skip {
       "OMIT" |
       "BACKWARD" |
       "FORWARD"
   }

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Acknowledgments

   Thanks to the following for feedback: Mark Davis, Mike Douglass,
   Donald Eastlake, Peter Edberg, Marten Gajda, Philipp Kewisch, Barry
   Leiba, 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.

Authors' Addresses

   Cyrus Daboo
   Apple Inc.
   1 Infinite Loop
   Cupertino, CA  95014
   United States

   EMail: cyrus@daboo.name
   URI:   http://www.apple.com/

   Gregory Yakushev
   Google Inc.
   Brandschenkestrasse 100
   8002 Zurich
   Switzerland

   EMail: gyakushev@yahoo.com
   URI:   http://www.google.com/

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