Guides · Updated 2026-10-26

What is an .ics file and how does it work?

What is an .ics file?

An .ics file is a text file in the iCalendar format, defined in RFC 5545 (September 2009). Google Calendar, Outlook, Apple Calendar and most other calendar apps can import it. Its media type is text/calendar and its default character set is UTF-8 (section 3.1.4).

The file is a set of nested blocks. A VCALENDAR wraps everything, and each event is a VEVENT inside it. Other components include VTODO (tasks) and VTIMEZONE (time zone rules).

What does a minimal valid .ics file look like?

This is a complete one-hour event at 09:00 UTC on 14 November 2026:

BEGIN:VCALENDAR
VERSION:2.0
PRODID:-//Example Org//Event Page//EN
BEGIN:VEVENT
UID:20261114-launch@example.com
DTSTAMP:20261026T120000Z
DTSTART:20261114T090000Z
DTEND:20261114T100000Z
SUMMARY:Product launch webinar
LOCATION:https://example.com/live
DESCRIPTION:Join the live stream.\nSlides are sent afterwards.
END:VEVENT
END:VCALENDAR

Three formatting rules trip up hand-written files:

• Line endings are CRLF (\r\n), per section 3.1. Files saved with bare LF line endings are technically invalid, even though many apps accept them.
• Lines should not exceed 75 octets. Longer lines are folded by inserting CRLF followed by a space or tab (section 3.1).
• Text escaping. In text values, commas, semicolons and backslashes are escaped with a backslash, and a line break is written as \n (section 3.3.11).

Which fields are required in an .ics event?

The RFC’s grammar sets a short list of required properties:

Keep the UID stable for an event. Calendar apps use it to recognise the same event when they receive it again; a new UID creates a duplicate.

How do .ics files handle all-day events?

Use date values with no time: DTSTART;VALUE=DATE:20261114. The end is non-inclusive, so a one-day event on 14 November has DTEND;VALUE=DATE:20261115. If DTEND is left out, RFC 5545 says a date-only event lasts one day (section 3.6.1). Setting DTEND to the same day as DTSTART is a common bug.

How do time zones work in .ics files?

A date-time can take three forms (section 3.3.5):

If you use TZID, the RFC requires a matching VTIMEZONE block in the same file for each TZID value (section 3.2.19). Some apps resolve common names such as Europe/Berlin without it, but not all do.

UTC is the safest choice for one-off events. For a weekly 10:00 meeting, use TZID and VTIMEZONE: a UTC start would move by an hour for attendees when daylight saving time changes. Avoid floating times unless the event should happen at the same clock time everywhere, such as “take medication at 08:00”.

How do recurring events work in .ics files?

Add an RRULE (section 3.8.5.3) to the VEVENT. For example, every Tuesday for ten weeks:

RRULE:FREQ=WEEKLY;BYDAY=TU;COUNT=10

Use COUNT or UNTIL to end the series, not both: the RFC says they must not appear in the same rule. Skip individual dates with EXDATE.

Why don’t changes to an .ics file reach people who already added it?

Because opening a downloaded or emailed .ics file imports a copy. The calendar keeps no link to the source. Microsoft’s Outlook.com help says the calendar “doesn’t refresh the imported events automatically”, even when the owner makes an update.

There are two ways to change an event after people have added it:

• Subscribed feeds. Publish the .ics at a fixed URL and have people subscribe, often via a webcal:// link. The calendar polls the URL: Microsoft says Outlook.com checks about every 3 hours (sometimes more than 24) and Outlook on the web about every 6 hours. Google Calendar’s “From URL” option only works from a computer browser, not the mobile apps.
• Invitations. Send the event as a scheduling message (METHOD:REQUEST, defined in RFC 5546) with the same UID and a higher SEQUENCE. This is how meeting updates and cancellations work, and it requires a named organizer and attendees.

Some hosted add-to-calendar services handle the subscription feed for you. See the comparison table for which ones do, or hand-built links and .ics files for the free route.

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