A.1.1. History

The iCalendar specification was first produced by the IETF in 1998 as IETF RFC 2445. Since then it has become the dominant standard for calendar data interchange on the internet and between devices (desktop computers, mobile phones etc.). The specification was revised in 2009 as IETF RFC 5545.

Alongside iCalendar is the iTIP specification (IETF RFC 2446 and revised as IETF RFC 5546) that defines how iCalendar is used to carry out scheduling operations (for example, how an organizer can invite attendees to a meeting and receive their replies). This forms the basis for email-based scheduling using iMIP (the specification that describes how to use iTIP with email — IETF RFC 6047).

iCalendar itself is a text-based data format. However, an XML format is also available, providing a one-to1034 one mapping to the text format (Internet-Draft draft-daboo-et-al-icalendar-in-xml-11).

iCalendar data files typically have a .ics file name extension. Most desktop calendar clients can import or export iCalendar data, or directly access such data over the Internet using a variety of protocols.