Short answer
For everyday purposes UTC and GMT show the same time. The difference is what they are: UTC (Coordinated Universal Time) is the atomic time standard every clock is set from, while GMT (Greenwich Mean Time) is a time zone, the UK's winter time. UTC never changes. The UK leaves GMT in summer and is on GMT (UTC+0) right now.
So the question is rarely "what time is it in each?" and almost always "which word should I write?". The answer depends on whether you mean a fixed reference or a place.
Two names, two jobs
UTC is a standard. It is calculated from atomic clocks in national laboratories around the world and coordinated by the International Bureau of Weights and Measures (BIPM) near Paris. It belongs to no country. Every time zone on Earth is defined as an offset from it, such as UTC+5:30 for India or UTC−5 for New York in winter. UTC never has daylight saving.
GMT is a zone, with a history as a standard. Greenwich Mean Time began as the mean solar time at the Royal Observatory in Greenwich, London. In 1884 the International Meridian Conference in Washington chose the Greenwich meridian as zero longitude, and for decades GMT was the world's reference time. Today GMT is best understood as the name of a time zone at UTC+0: the UK and Ireland in winter, and places such as Iceland, Ghana and Senegal all year.
That is why the two names show the same hour, yet only one of them is safe to use as a fixed reference.
How far apart are they?
In practice, not at all. Any clock, phone or computer set to GMT shows the same time as one set to UTC.
Strictly, the old astronomical GMT and atomic UTC are different time scales. Atomic clocks tick perfectly evenly, while the Earth's rotation speeds up and slows down slightly. To stop atomic time drifting away from the sun, UTC has leap seconds added when needed, so it never strays more than 0.9 seconds from time based on the Earth's rotation. The gap between astronomical GMT and UTC is therefore always under a second. That matters to astronomers and satellite engineers, not to anyone booking a meeting.
UTC and GMT side by side
| UTC | GMT | |
|---|---|---|
| Full name | Coordinated Universal Time | Greenwich Mean Time |
| What it is | The world's time standard | A time zone (and a historic standard) |
| Based on | Atomic clocks, with leap seconds | Mean solar time at Greenwich |
| Kept by | BIPM, from national laboratories | Today taken from UTC |
| Daylight saving | Never | GMT itself does not change, but the UK leaves it in summer |
| Offset | The zero point | UTC+0 |
| Typical users | Aviation, computing, science, international standards | UK and Irish winter time, West Africa, Iceland, everyday speech |
The glossary entries for UTC and GMT list the zones that use each one.
The summer trap
The most common mistake is writing "GMT" when you mean "UK time". The UK only uses GMT from late October to late March. For the rest of the year it is on BST (British Summer Time), which is UTC+1.
So "10:00 GMT" in July is not 10:00 in London. It is 11:00 in London, and anyone who converts it correctly will turn up an hour late by London's clock. The UK's next clock change is on Sunday 28 March 2027; the UK daylight saving page has the full schedule.
If you mean "whatever the clock in London says", write "UK time" or "London time", or name the right abbreviation for the season. The London clock always shows which one is in force. If you mean a fixed reference that never moves, write UTC.
Which one to use when
Software, servers and databases: UTC. Store and log times in UTC and convert to local time only for display. Programming languages, operating systems and the IANA time zone database all treat UTC as the reference.
Aviation, shipping and the military: UTC, usually called Zulu time. Our guide to Zulu time explains the Z and how to convert it.
International meetings and invites: UTC, or a city. "15:00 UTC" means one moment for everyone. "15:00 GMT" invites confusion in summer. Better still, give the time in each attendee's city; the meeting planner does that for you.
Talking to people in the UK in winter: GMT is fine. It is the correct name for UK time between the autumn and spring clock changes.
UK law: GMT. Acts of Parliament refer to Greenwich mean time, adjusted by the Summer Time Act for the summer months.
Offsets: either, but be consistent. "UTC+5:30" and "GMT+5:30" mean the same thing. UTC is now the more common and more precise way to write it.
A note for developers: the reversed signs
The IANA time zone database contains zones named Etc/GMT+5 and Etc/GMT-3. Their signs are inverted from what people expect: Etc/GMT+5 is five hours behind UTC, the same as UTC−5. This follows an old POSIX convention. Unless you have a reason to use a fixed offset, pick a real location such as Europe/London instead, which also handles daylight saving correctly.
To check a specific conversion, use our GMT to UTC converter, or see the live clock on the UTC+0 page.
Frequently asked questions
Is GMT the same as UTC?
They show the same time, so for daily life you can treat them as equal. UTC is the atomic standard; GMT is a time zone at UTC+0 and an older astronomical time scale. They never differ by more than a second.
Is UK time GMT or UTC?
Neither, all year round. The UK is on GMT (UTC+0) in winter and BST (UTC+1) in summer. Right now it is on GMT.
Is GMT+0 the same as UTC+0?
Yes. Both mean no offset from the world's reference time.
Should I write GMT or UTC in a meeting invite?
Write UTC if you want a fixed reference, or the city name if you mean local time. Avoid GMT in summer, when the UK is not on it.
Does UTC change in summer?
No. UTC never has daylight saving. Places change their offset from UTC instead.