Time standards · 3 min read
UTC vs GMT: what is the difference?
A clear guide to the international time standard, Greenwich Mean Time and why they are not quite interchangeable.
A London train timetable, an aviation weather report and a world clock may all appear to show the same hour. But the labels GMT and UTC describe different things. Knowing which is which makes scheduling and time-zone conversion much less mysterious.
One reference, two histories
Coordinated Universal Time (UTC) is the international reference time scale. National laboratories maintain atomic clocks, and the International Bureau of Weights and Measures (BIPM) computes UTC from their contributions. It is a standard for agreeing on when an instant happens, not the name of a city or a daylight-saving schedule.
Greenwich Mean Time grew from observations of the Sun near the Greenwich meridian. Today GMT is also used as a civil time-zone label, notably for the United Kingdom in winter. In summer, London uses British Summer Time (BST), one hour ahead of UTC. A date matters: saying ‘London is GMT’ year-round is incorrect.
Why the numbers often match
When a location is on GMT, its wall clock normally reads the same hour and minute as UTC. That does not make the concepts identical: UTC is the globally coordinated reference, while GMT describes a local civil convention or a historical solar-time idea. A meeting at 15:00 UTC names an unambiguous instant; a meeting at 15:00 London time needs a date to determine whether London is on GMT or BST.
For travel or distributed work, keep the event as an instant in UTC, then display it in each participant’s IANA time zone, such as Europe/London. Avoid storing ‘GMT+1’ as if it were a place: political rules and summer changes can alter the offset.
A practical conversion
Suppose an online event is at 12:00 UTC in July. London is then usually on BST, so its clock reads 13:00. In January, the same 12:00 UTC event reads 12:00 in London. The UTC instant is stable; only the displayed local time changes. Use the date-aware converter rather than subtracting a fixed number of hours.
You may also see UT1, an astronomical measure related to Earth’s rotation, in technical writing. UTC is kept close to UT1 through international timekeeping decisions, but everyday appointments and software generally use UTC as the reference.
Sources & further reading
These sources provide the underlying standards or research. This guide is educational and is not medical advice.