September 15, 2026 6 min read

UTC vs GMT: What Is the Difference?

On a clock, UTC and GMT show exactly the same time. The difference is what they are. UTC (Coordinated Universal Time) is the world's time standard, kept by atomic clocks. GMT (Greenwich Mean Time) is a time zone, used by the UK in winter and by Iceland and much of West Africa all year. Most of the time you can treat them as equal. Three situations are the exception, and one of them catches people every summer.

Short answer

For everyday use, 14:00 UTC and 14:00 GMT are the same moment. Use UTC when you mean the global reference that every time zone is measured from. Use GMT only when you mean the local time of a place that is on GMT, and remember that the UK is not on GMT between late March and late October.

Where it gets slippery: "London time" and "GMT" are not interchangeable. On September 15, London is on British Summer Time, one hour ahead of both UTC and GMT. A meeting set for "3 p.m. GMT" in summer is, strictly, 4 p.m. in London.

What GMT is

Greenwich Mean Time started as an astronomical measurement. Royal Museums Greenwich defines it as the yearly average of the time each day when the Sun crosses the Prime Meridian at the Royal Observatory. "Mean" is the key word: real solar days vary in length across the year, so astronomers averaged them into equal clock days.

British railways adopted Greenwich time in the 1840s so their timetables would agree, and it became Britain's legal standard time in 1880. In 1884 the International Meridian Conference in Washington recommended the Greenwich meridian as the world's prime meridian, and time zones came to be defined as whole-hour offsets from GMT. From 1884 until 1972, GMT served as the international standard of civil time.

Today GMT survives as a zone name. It is UK legal time in winter, and the Europe/London time zone shows the abbreviation GMT from late October to late March. Reykjavik uses GMT all year, as do Accra, Dakar and Abidjan.

What UTC is

Coordinated Universal Time is computed by the International Bureau of Weights and Measures (BIPM) from atomic clocks in national laboratories around the world, and each lab broadcasts its own realization of it (NIST's is called UTC(NIST)). Its second is the SI second, defined by the cesium atom, not by the Earth's rotation. UTC took its current form in 1972.

Atomic seconds are steady; the Earth is not. Its rotation wobbles and, over the long term, slows, so a purely atomic clock would slowly drift away from the Sun. UTC handles this with leap seconds. Whenever the gap between UTC and UT1, the time scale based on the Earth's actual rotation, threatens to reach 0.9 seconds, an extra second is added: 23:59:59, then 23:59:60, then 00:00:00.

Twenty-seven leap seconds were inserted between 1972 and 2016. The last one came on December 31, 2016, and none has been needed since, because the Earth has been turning slightly faster in recent years. NIST's weekly figures show UT1 minus UTC falling from about +75 milliseconds in January 2026 to a few milliseconds by early September. In 2022 the General Conference on Weights and Measures decided to raise the 0.9-second limit by 2035, which is meant to keep UTC free of jumps for at least a century; we cover that in leap seconds and why they are ending.

For the longer explanation of how UTC is built and why every offset is written against it, read what UTC is.

Comparison table

When the difference matters

1. The UK in summer

This is the common mistake. From the last Sunday in March to the last Sunday in October, the UK is on British Summer Time, UTC+1. Writing "GMT" for London during those months gives the wrong hour, and so does assuming that a UK colleague who says "GMT" means UTC+0. In 2026, BST runs from March 29 to October 25. Check the current time in London when in doubt.

2. Software zone names with flipped signs

The IANA time zone database includes fixed-offset zones such as Etc/GMT+5. They follow the old POSIX convention, where positive means west of Greenwich, so Etc/GMT+5 is UTC-5 (New York's winter offset) and Etc/GMT-3 is UTC+3. The tz data file itself warns that this is "the opposite of what many people expect". For a fixed offset in code, use "UTC" for zero and a geographic zone like America/New_York for everything else.

There is a related trap in time zone pickers: a list that labels zones "GMT+1" can be showing the current offset, which changes with daylight saving time, or a fixed one. A real place name is always safer than an offset label.

3. Sub-second precision

If you are navigating by the stars, pointing a telescope or doing satellite geodesy, the difference between atomic time (UTC) and Earth-rotation time (UT1, the modern descendant of astronomical GMT) matters. It has always stayed within 0.9 seconds, and NIST radio stations broadcast the current correction, called DUT1, in 0.1-second steps. For anyone else, that gap is invisible.

Which one to write

Our recommendation is simple. Write UTC for anything international, technical or legal: server logs, flight plans, API timestamps, a meeting with people in five countries. Write GMT or BST only when you mean UK local time, and pick the one that is actually in force on that date. If you are unsure which, write "UK time" or "London time" and let the reader's calendar sort it out.

In ISO 8601 timestamps, UTC is written with a Z suffix, as in 2026-09-15T18:51Z, which is unambiguous in every country. The UTC page and the GMT page on this site show the current time in each, and the world clock puts London, UTC and your own zone side by side so you can see whether they match today.

Frequently asked questions