UTC vs GMT: what's the difference?
UTC and GMT show the same time, so for everyday purposes they're interchangeable. The difference is what they are: UTC is the world's atomic time standard, kept within 0.9 seconds of Earth's rotation by leap seconds, while GMT is a time zone used by the UK and some African countries, and an older astronomical standard.
Key facts
| Clock difference between UTC and GMT | None (UTC is kept within 0.9 s of rotation-based time) |
|---|---|
| UTC is | A time standard, based on atomic clocks |
| GMT is | A time zone, originally mean solar time at Greenwich |
| UTC introduced | 1960s; current form with leap seconds from 1972 |
| UK in summer | BST = UTC+1, not GMT |
| Difference between UTC and atomic time (TAI) | 37 seconds |
| Used by aviation and the military | UTC, called 'Zulu time' |
The short version
If a schedule says 14:00 GMT or 14:00 UTC, it means the same moment. Where the difference matters is in summer and in software.
| UTC | GMT | |
|---|---|---|
| What it is | Global time standard | Time zone (and historic standard) |
| Based on | Atomic clocks, adjusted with leap seconds | Earth's rotation, measured at Greenwich |
| Changes for daylight saving? | Never | No, but the UK switches from GMT to BST in summer |
| Who uses it | Computers, aviation, science, the internet | UK and Ireland in winter, Iceland, parts of West Africa |
A common mistake
People often say "London is on GMT." In winter it is. From the last Sunday in March to the last Sunday in October, London is on British Summer Time, which is UTC+1. A meeting "at 9:00 GMT" in July is really at 10:00 London time. When precision matters, write UTC with an offset, like "09:00 UTC."
Where the 37 seconds comes in
You may see the number 37 seconds attached to UTC. That's the gap between UTC and International Atomic Time (TAI), which never adds leap seconds. Since 1972, 27 leap seconds have been added to UTC, on top of a starting offset of 10 seconds, so UTC is now 37 seconds behind TAI. It is not a difference between UTC and GMT. See leap seconds.
Why UTC replaced GMT
GMT was based on the sun's mean position over Greenwich, which depends on Earth's slightly irregular rotation. Once atomic clocks proved more stable than the planet, scientists needed a standard built on them, and UTC took over that job. "GMT" survives as the name of a time zone and in everyday speech.
Questions people ask
Is UTC the same as GMT?
On the clock, yes; they show the same time. Technically, UTC is an atomic time standard and GMT is a time zone.
Is the UK on GMT or UTC?
The UK uses GMT (equal to UTC) in winter and BST (UTC+1) in summer.
Why is it called UTC and not CUT?
It's a compromise between the English 'Coordinated Universal Time' (CUT) and the French 'Temps Universel Coordonné' (TUC).
Sources
Last updated October 18, 2026. Found an error? Tell us.