http://www.rfc-editor.org/rfc/rfc7529.txt
RFC 7529: Non-Gregorian Recurrence Rules in the Internet Calendaring and Scheduling Core Object Specification (iCalendar), C. Daboo, G. Yakushev2015-05-01T23:00:00-00:00This 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.