iCalendar (.ics) format

The iCalendar standard (RFC 5545) defines a text format for calendar data supported by Google Calendar, Apple Calendar, Outlook, and virtually every other calendar application. Files use the .ics extension.

VEVENT structure

BEGIN:VCALENDAR
VERSION:2.0
PRODID:-//YourApp//EN
BEGIN:VEVENT
UID:unique-id@yourdomain.com
DTSTAMP:20240101T120000Z
DTSTART:20240615T090000Z
DTEND:20240615T100000Z
SUMMARY:Team Meeting
DESCRIPTION:Weekly sync
LOCATION:Conference Room A
END:VEVENT
END:VCALENDAR

iCalendar properties reference

Property	Description	Example
`DTSTART`	Event start date/time	`20240615T090000Z`
`DTEND`	Event end date/time	`20240615T100000Z`
`SUMMARY`	Event title	`SUMMARY:Team Meeting`
`DESCRIPTION`	Detailed description (use \n for line breaks)	`DESCRIPTION:Agenda item 1\nItem 2`
`LOCATION`	Location string or URL	`LOCATION:Conference Room A`
`RRULE`	Recurrence rule	`RRULE:FREQ=WEEKLY;BYDAY=MO`
`VALARM`	Reminder component	`TRIGGER:-PT15M` (15 min before)
`UID`	Unique event identifier	Should be globally unique

Recurring events (RRULE)

The RRULE property defines repeating patterns using a structured syntax:

RRULE:FREQ=DAILY;COUNT=5 - repeat daily for 5 occurrences
RRULE:FREQ=WEEKLY;BYDAY=MO,WE,FR - every Monday, Wednesday, Friday
RRULE:FREQ=MONTHLY;BYDAY=1MO - first Monday of each month
RRULE:FREQ=YEARLY;UNTIL=20261231T000000Z - annually until December 31, 2026

Timezone handling

Use the TZID parameter with a standard IANA timezone name rather than converting all times to UTC. This prevents Daylight Saving Time ambiguity:

DTSTART;TZID=America/New_York:20240615T090000

Using UTC (Z suffix) is simpler but means the event will shift by one hour in local time when DST transitions occur. TZID-based times always represent the correct local time regardless of clock changes.

Key date format

Dates use ISO 8601: YYYYMMDD for all-day events; YYYYMMDDTHHmmSSZ for timed events (Z = UTC). Omit Z and add a TZID for local time zones.