GMT vs UTC: What’s Actually the Difference?

Ask what time it is in London in winter and you might hear "GMT". Ask a server, a pilot or a programmer and you'll hear "UTC". The two agree to the second in everyday life — yet they are fundamentally different kinds of time.

GMT: time from the sky

Greenwich Mean Time is mean solar time at the Royal Observatory in Greenwich, London — time defined by the Earth's rotation, averaged over the year to smooth out the planet's slightly elliptical orbit. From the 1884 Meridian Conference until the mid-20th century, GMT was the world's reference time.

UTC: time from atoms

Coordinated Universal Time, refined into its modern form by 1972, is built the other way around: it ticks with the metronomic regularity of caesium atomic clocks, and historically stayed within 0.9 seconds of Earth-rotation time by occasionally inserting a leap second. UTC is the basis of civil timekeeping everywhere — every time zone is defined as an offset from it, GPS and the internet run on it, and "GMT" survives mostly as the name of the UTC+00:00 winter zone used by the UK, Ireland, Iceland and parts of West Africa.

So which should you use?

In conversation, either is fine — the difference is well under a second. In anything technical, say and store UTC: it is the unambiguous standard, written with a Z suffix in ISO 8601 (14:30Z) and called "Zulu time" in aviation. And remember that London is only on GMT half the year — in summer it moves to BST, UTC+1, a classic scheduling trap. When it matters, convert with the time zone converter or compare zones side by side with the world clock.