UTC vs GMT: What's the Difference? (Plain-English Guide)
By WordTimezone · Updated October 7, 2026 · 3 min read
UTC and GMT show the same clock time, which is why they are used interchangeably. But they are not the same thing: GMT is a time zone (and an old astronomical standard), while UTC is the modern time standard that every other zone is measured from. The distinction matters when you write software, schedule international events, or explain why London is not on “GMT” in summer.
The short answer
| GMT | UTC | |
|---|---|---|
| Full name | Greenwich Mean Time | Coordinated Universal Time |
| What it is | A time zone, based on mean solar time at the Royal Observatory in Greenwich | A time standard kept by atomic clocks |
| Leap seconds | Not applicable | Added occasionally to stay close to Earth’s rotation |
| Observes daylight saving | Used as the winter time of the UK and Ireland | Never — it has no daylight saving |
| Best used for | Describing a zone (e.g. “GMT+1”) | Software, aviation, science, global event times |
Why they are almost the same
GMT was the world’s reference for timekeeping for a long time: noon was when the sun crossed the meridian through Greenwich. As atomic clocks became more precise than the spinning Earth, scientists defined UTC, which ticks at the steady rate of atomic time but is occasionally nudged with a leap second so it never drifts more than 0.9 seconds from the Earth’s rotation. Since 1972, 27 leap seconds have been added, the most recent at the end of 2016. In 2022 the international metrology community agreed to stop using leap seconds by 2035.
For everyday scheduling the difference is below a second, so “UTC+0” and “GMT” are the same clock time. The confusion comes from what people do with the name GMT.
London is not on GMT in summer
The UK uses GMT in winter and British Summer Time (BST, UTC+1) in summer. So saying “3 PM GMT” for a July meeting in London is a mistake — local time is then UTC+1. A few places stay on UTC+0 all year:
| Place | IANA zone | UTC offset (Jan / Jul 2026) |
|---|---|---|
| London (UK) | Europe/London | UTC / UTC+1 |
| Dublin (Ireland) | Europe/Dublin | UTC / UTC+1 |
| Lisbon (Portugal) | Europe/Lisbon | UTC / UTC+1 |
| Reykjavík (Iceland) | Atlantic/Reykjavik | UTC |
| Accra (Ghana) | Africa/Accra | UTC |
Iceland and Ghana are on UTC+0 all year; the UK, Ireland and Portugal move to UTC+1 in summer.
Which should you write?
- Global events, logs, APIs, flight and server times: UTC. In ISO 8601 write
2026-10-07T14:00:00Z(the “Z” means UTC). - Describing a zone: use an offset (“UTC+5:30”) or a city, not “GMT+…”.
- UK-specific communication: say “UK time” or “London time” so the season does not matter.
Converting from UTC
To convert from UTC, add or subtract the zone’s offset. India is UTC+5:30, so 14:00 UTC is 7:30 PM IST. Use the UTC to IST converter or IST to UTC to check, and see the UK’s time zone facts for its clock-change dates.
Frequently asked questions
Is UTC the same as GMT?
For everyday clock time, yes — UTC+0 and GMT show the same time. Technically GMT is a time zone based on solar time at Greenwich, while UTC is an atomic-clock standard with leap seconds.
Is London on GMT in summer?
No. London uses British Summer Time (UTC+1) from the last Sunday in March to the last Sunday in October, and GMT in winter.
Should I use UTC or GMT in software?
Use UTC, and store times as UTC (for example in ISO 8601 with a Z). Use a named IANA zone such as Europe/London when you need local time.
Why does UTC have leap seconds?
The Earth’s rotation is slightly irregular. Leap seconds were added occasionally to keep UTC within 0.9 seconds of Earth-rotation time; the practice is due to end by 2035.