UTC vs GMT

For checking the time or planning a call, UTC and GMT are the same. At any moment they show the same hour and minute, and a time written as UTC+2 means the same as GMT+2.

The difference is what each name means. UTC is the world's time standard, kept by atomic clocks. GMT is the older, sun-based time it grew out of, and today it's mostly used as the name of a time zone: the one at 0° longitude, used all year in Iceland and in winter in the UK and Ireland.^[1]

Greenwich Mean Time began as the time kept at the Royal Observatory in Greenwich, London. "Mean" means average: noon by the sun drifts a little through the year, so astronomers used the average time the sun crossed the Greenwich meridian.^[2]

In 1884 the Greenwich meridian was chosen as 0° longitude for the world, and GMT became the reference that other time zones were measured from.^[2] For most of the twentieth century, "GMT+1" or "GMT−5" was how you described a time zone.

The Earth isn't a perfect clock. Its spin speeds up and slows down very slightly, so time based on the sun wanders. Atomic clocks don't. UTC, Coordinated Universal Time, is the international time standard, calculated from atomic clocks around the world.^[3]

To stop atomic time drifting away from the sun, UTC has occasionally added a leap second, an extra second at the end of June or December.^{[3, 4]} There have been 27 since 1972, and the last one was added at the end of 2016.^[4] In 2022, the world's measurement authorities decided to stop adding them by 2035 and let UTC drift further from the sun before any correction.^[5]

So UTC and time by the sun have always stayed within a second of each other.^[5] For everyday life, that difference doesn't exist.

In English, Coordinated Universal Time would shorten to CUT. In French, temps universel coordonné would be TUC. To avoid favoring either language, the international experts who created it in 1970 picked UTC for every language.^[1]

The two names aren't always interchangeable. Watch out for these:

• "GMT" in the UK isn't year-round. UK law sets legal time as Greenwich mean time, but in summer the country moves its clocks an hour ahead, to British Summer Time.^[6] From late March to late October, London is UTC+1, not GMT. If someone says "3 PM GMT" in July, check whether they mean London time or actual GMT.
• Science, aviation and computing use UTC. Pilots, ships and the military often call it "Zulu" time and write it with a Z, as in 1430Z.^[1] Computers store times in UTC because it never has daylight saving changes.
• Precise timekeeping is based on UTC. Anything that depends on exact seconds, like satellite navigation, telecoms networks or financial trading records, works from UTC and its atomic clocks, not from the sun. That's also why leap seconds cause trouble for these systems.^[5]

Use UTC when you need one reference time for people in several places, for example in a meeting invitation, a status page or a log file. It never changes with the seasons, so "14:00 UTC" means the same moment all year.

Use GMT when you mean the time zone itself, or UK winter time. And when you mean "the time in London", it's clearer to just say London.

To see how offsets from UTC turn into local times, read how time zones work, or open the world clock to see UTC next to your own time.

1. 1.NIST, How is UTC(NIST) related to UTC, TAI, GMT, GPS time and Zulu time?. GMT as today's name for the zone at 0°, Zulu time, and why the abbreviation is UTC.
2. 2.Royal Museums Greenwich, Greenwich Mean Time (GMT). GMT as mean solar time at Greenwich, and the 1884 choice of the Greenwich meridian.
3. 3.BIPM, Time metrology. UTC as the international reference time scale, built from atomic time with leap seconds.
4. 4.IANA, leap-seconds.list. The 27 leap seconds added since 1972, the last at the end of 2016.
5. 5.BIPM, Resolution 4 of the 27th CGPM (2022). The decision to let UTC drift further from the Earth's rotation by 2035.
6. 6.legislation.gov.uk, Interpretation Act 1978, section 9. Greenwich mean time as the UK's legal time, subject to summer time.