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

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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
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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Daboo & Yakushev             Standards Track                    [Page 1]
RFC 7529               iCalendar RSCALE Extension               May 2015

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