



Network Working Group                                           C. Daboo
Internet-Draft                                                Apple Inc.
Updates: 5545, 6321, 7265 (if approved)                      G. Yakushev
Intended status: Standards Track                             Google Inc.
Expires: August 15, 2015                               February 11, 2015


              Non-Gregorian Recurrence Rules in iCalendar
                      draft-ietf-calext-rscale-04

Abstract

   This document defines extensions to iCalendar (RFC 5545) to support
   use of non-Gregorian recurrence rules.  It also defines how CalDAV
   (RFC 4791) servers and clients can be 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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   Internet-Drafts are draft documents valid for a maximum of six months
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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 August 15, 2015.

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
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   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  . . . . . . . . . . . . . . . . . . . . . . . .   9
   5.  Registering Calendar Systems  . . . . . . . . . . . . . . . .  12
   6.  Compatibility . . . . . . . . . . . . . . . . . . . . . . . .  13
   7.  Use with iTIP . . . . . . . . . . . . . . . . . . . . . . . .  13
   8.  Use with xCal . . . . . . . . . . . . . . . . . . . . . . . .  14
   9.  Use with jCal . . . . . . . . . . . . . . . . . . . . . . . .  15
   10. Use with CalDAV . . . . . . . . . . . . . . . . . . . . . . .  16
     10.1.  CALDAV:supported-rscale-set Property . . . . . . . . . .  17
   11. Security Considerations . . . . . . . . . . . . . . . . . . .  17
   12. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  17
   13. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  17
   14. References  . . . . . . . . . . . . . . . . . . . . . . . . .  18
     14.1.  Normative References . . . . . . . . . . . . . . . . . .  18
     14.2.  Informative References . . . . . . . . . . . . . . . . .  18
   Appendix A.  Change History (To be removed by RFC Editor before
                publication) . . . . . . . . . . . . . . . . . . . .  19
   Appendix B.  xCal RELAX NG schema update  . . . . . . . . . . . .  21
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  22

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.




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   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 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 fixed to the Gregorian calendar system, and
       another (a "VEVENT") wants to use a different non-Gregorian
       calendar scale, and 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 re-defining 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, 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
   [UNICODE.ICU].

   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",
   "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 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.  The "{}" prefix is used for purely illustrative
   purposes and never appears in actual date-time values 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.  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.



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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 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:

   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 value 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 /=   ("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 a 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 re-applied 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 "BYMONTHDAY" and "BYMONTH" elements in an
       "RRULE" have 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 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.
   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.

4.3.  Examples

4.3.1.  Chinese New Year

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




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   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 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 the one in Gregorian year 2013.  Whilst 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]).

   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:








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               +---------------+--------------------------+
               | 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
   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                 |
                +--------------+--------------------------+








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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 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
   that:

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

   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.  "RSCALE" values are case-insensitive, but upper case is
   preferred.



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   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 fallback 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.

   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 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.




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   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
      [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.

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.





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   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 B.

   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.

   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:






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   [
     "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 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 (as per option (2) in Section 6).





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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>

11.  Security Considerations

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

12.  IANA Considerations

   This document requires no IANA actions.

13.  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.



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

14.  References

14.1.  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
              2009.

   [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.

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

14.2.  Informative References

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




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

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

   Changes in draft-ietf-calext-rscale-04:

   1.  AD review: Clarified that {} prefix is purely illustrative.

   2.  AD review: reworded SKIP formal syntax comment.

   3.  AD review: clarified mapping of ICU to iCalendar months for
       Hebrew calendar.

   4.  AD review: tweaked introductory text of Chinese example.

   5.  AD review: changed Ethiopic example to use MONTHLY and explained
       why that rule was chosen.

   6.  AD review: removed "SHOULD" for RSCALE upper case.

   7.  AD review: "SKIP" - added new section, changed the default value.

   8.  OPSDIR review: fixed Ethiopic example.

   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
       appendix.

   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".




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   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
       attributes.

   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.




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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-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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Authors' Addresses

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

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


   Gregory Yakushev
   Google Inc.
   Brandschenkestrasse 100
   8002 Zurich
   Switzerland

   Email: yakushev@google.com
   URI:   http://www.google.com/































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