When someone sends you a meeting invite, a booking confirmation or an event ticket, it often comes with a small attachment ending in .ics. Opening it adds the event to your calendar. ICS files are plain text in a standard format called iCalendar, defined in RFC 5545, and almost every calendar app understands them.
What is inside an ICS file
Open one in a text editor and you will see something like this:
BEGIN:VCALENDAR
VERSION:2.0
PRODID:-//Example Corp//Calendar//EN
BEGIN:VEVENT
UID:20261205-craft-market@example.com
DTSTART;TZID=Europe/London:20261205T100000
DTEND;TZID=Europe/London:20261205T160000
SUMMARY:Winter craft market
LOCATION:St Mark's Hall
DESCRIPTION:Free entry.
END:VEVENT
END:VCALENDAR
Key parts:
- BEGIN:VEVENT / END:VEVENT wrap a single event.
- UID is a unique ID, so updates replace the old event instead of duplicating it.
- DTSTART and DTEND are the start and end, with a time zone.
- SUMMARY, LOCATION and DESCRIPTION are the title, place and notes.
How to open an ICS file
- Windows: double-click to open in Outlook or the Calendar app, then save.
- Mac: double-click to add it to Apple Calendar.
- Google Calendar: go to Settings → Import & export → Import and choose the file. Opening the attachment in Gmail usually offers an "Add to calendar" button too.
- iPhone and Android: tap the attachment and choose Add to calendar.
To read the contents without importing, open it in any plain text editor. The Docento.app text editor opens text files in the browser.
Why an invite shows the wrong time
The most common ICS problem is time zones:
- A time ending in Z (such as
20261205T100000Z) is in UTC, and your calendar converts it to local time. - A time with TZID uses that named zone.
- A time with neither is "floating" and shows the same clock time everywhere, which goes wrong for people in other zones.
Daylight saving changes add a second layer of confusion in late October and early November, when different regions change clocks on different dates.
Why an invite creates duplicates
If the organiser sends an update with a different UID, calendars treat it as a new event. Correct updates keep the same UID and increase a SEQUENCE number.
Editing an ICS file
Because it is plain text, you can fix small errors by hand, such as a typo in the title or a wrong location. Keep the structure intact: every BEGIN needs a matching END, and long lines are folded with a leading space on continuation lines. For other plain-text formats, see plain text vs rich text files.
Takeaway
An ICS file is a plain-text calendar event in the iCalendar format. Double-click or import it to add the event, open it in a text editor to read it, and check the DTSTART time zone when an invite shows the wrong time.
