Is UTC the same as GMT?
For an everyday calendar invitation, UTC and GMT show the same clock reading: zero hours of offset. But the names describe different things. UTC is the international reference time standard. GMT is a time-zone name with historical roots in mean solar time at Greenwich.
The distinction that catches people out is simpler: London is not on GMT all year. During British Summer Time, London is one hour ahead of UTC. A meeting at 10:00 UTC is 10:00 in winter London and 11:00 in summer London.
UTC offsets vs time zones
An offset, such as UTC+01:00, tells you how far a clock reading is ahead of UTC at a particular moment. A location-based time zone, such as Europe/London, contains rules that determine the correct offset for a date.
That is why choosing “London” in a converter is different from choosing a fixed UTC+00:00 offset. One follows local seasonal rules; the other stays fixed.
UTC, GMT, BST, and Europe/London
| Name | Everyday meaning |
|---|---|
| UTC | Reference standard; no daylight-saving adjustment |
| GMT | A zero-offset time zone |
| BST | British Summer Time, UTC+1 in the UK context |
| Europe/London | London’s date-specific civil time rules |
What should go in an invitation?
For a one-off event, include a full date, a time, and either UTC or a named city. For a repeating local appointment, use a location-based time zone in your calendar so it can follow that city’s clock changes.
A useful format is “15 October, 10:00 London time.” A better one also includes the year and a calendar invite. Avoid a bare “10 GMT” when what you mean is “10 in London.”
Further reading
Sources: BIPM time metrology and the IANA time-zone database.