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A History of iCalendar

The iCalendar format has evolved over more than two decades through multiple standards iterations.

vCalendar (1996)

The predecessor to iCalendar was vCalendar (vCal), developed by the Versit Consortium (Apple, AT&T, IBM, Siemens) in 1996. It defined a text-based format for calendar data with vEvent and vTodo components.

iCalendar — RFC 2445 (1998)

The IETF standardized the format as RFC 2445. Key additions: standardized components (VEVENT, VTODO, VJOURNAL, VFREEBUSY, VTIMEZONE), recurrence rules (RRULE), alarm components, and formal ABNF grammar.

iCalendar — RFC 5545 (2009)

RFC 5545 obsoleted RFC 2445. It clarified ambiguities, deprecated rarely-used features, and improved interoperability requirements. This is the current standard.

  • RFC 5546 (iTIP) — Scheduling methods
  • RFC 6047 (iMIP) — Email transport for iTIP
  • RFC 6638 — CalDAV scheduling extensions
  • RFC 7808 — Time Zone Distribution Service
  • RFC 8984 (jsCalendar) — JSON calendar data
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