iCalendar

iCalendar is a widely used data format for exchanging calendaring and scheduling information between applications. It defines a text-based representation of calendar data capable of encoding events, tasks, journal entries, time zones, and alarms. Files typically use the .ics extension and are exchanged as attachments or via web links; the MIME type is text/calendar. The standard was developed by the Internet Engineering Task Force and published as RFC 5545, with RFC 5546 describing transport and scheduling messages (iTIP and iMIP). RFC 5545 supersedes the older RFC 2445.

An iCalendar document describes one or more CALENDAR components, including VEVENT for events, VTODO for to-dos, VJOURNAL for journal entries, and VALARM for alarms; VTIMEZONE defines time zones. Each component carries properties such as DTSTART, DTEND or DURATION, SUMMARY, DESCRIPTION, LOCATION, UID, CREATED, LAST-MODIFIED, SEQUENCE, STATUS, and optional properties like ATTENDEE, CATEGORIES, GEO, URL, and RECURRENCE RULE (RRULE) for recurrences. Calendars may also include multiple time zones via VTIMEZONE. Recurring events can specify exceptions (EXDATE) and exception instances (RECURRENCE-ID).

iCalendar is designed for interoperability across platforms; it underpins calendar sharing, invitations, and subscriptions. Popular calendar applications (Google Calendar, Apple Calendar, Microsoft Outlook, Mozilla Thunderbird) support ICS imports and exports and can subscribe to calendars using webcal URLs. While broadly compatible, implementation details can vary among applications.

In short, iCalendar provides a portable, standards-based syntax for encoding calendaring information to enable cross-platform exchange of events, tasks, and related scheduling data.