The short answer
For everyday purposes, GMT and UTC are the same: both are the time at the prime meridian, with an offset of zero. If it is 14:00 UTC, it is 14:00 GMT. The difference is in what each one is. GMT, Greenwich Mean Time, is a time zone name with astronomical roots, used as civil time in the UK in winter and in a number of countries all year. UTC, Coordinated Universal Time, is the international time standard, maintained with atomic clocks. Every time zone, including GMT, is defined as an offset from it.
A common source of confusion is UK time in summer. From the last Sunday in March to the last Sunday in October, UK clocks show British Summer Time (UTC+1), not GMT. On those dates, "London time" and "GMT" are an hour apart, even though GMT and UTC never are.
Where GMT came from
Before the railways, towns kept their own local time, set by the sun. Noon was when the sun was highest, so Bristol's clocks ran about ten minutes behind London's. Fast trains made this impractical. In 1840 the Great Western Railway adopted London time for its timetables, and other companies followed. "Railway time", based on Greenwich, spread across Britain over the next decade or so. In 1880 the Statutes (Definition of Time) Act made Greenwich Mean Time the legal time throughout Great Britain.
The Royal Observatory at Greenwich, founded in 1675, was the obvious reference. Its astronomers timed the moment stars crossed the meridian, the north–south line through the Airy transit circle installed in 1851. The word "mean" matters: real solar time drifts during the year because the Earth's orbit is elliptical and its axis is tilted, so GMT used an averaged, or mean, sun to give days of equal length.
In 1884 the International Meridian Conference in Washington DC recommended Greenwich as the prime meridian for the world. Time zones were later defined as whole or fractional hours east or west of it. Until 1925, astronomers counted the GMT day from noon. After that, the astronomical day was aligned with the civil day, starting at midnight.
Where UTC came from
By the mid-20th century, atomic clocks had shown that the Earth is not a perfect timekeeper. Its rotation varies slightly from year to year and is very gradually slowing over centuries. Since 1967 the second has been defined by a property of the caesium atom rather than as a fraction of a day. That left a choice: follow the atoms exactly and let clock time drift away from day and night, or follow the Earth and accept uneven seconds.
UTC is the compromise. It ticks in exact atomic seconds, but it is kept within 0.9 seconds of UT1, the time scale that tracks the Earth's actual rotation. UTC in its current form began on 1 January 1972. It is calculated by the International Bureau of Weights and Measures (BIPM) from the readings of hundreds of atomic clocks in national laboratories. The UK's contribution comes from the National Physical Laboratory. The pure atomic time scale behind UTC is called International Atomic Time (TAI).
The name is a bit of a puzzle: the abbreviation matches neither the English nor the French word order. UTC was chosen as a neutral form for all languages.
Leap seconds, and why they are ending
To keep UTC close to the Earth's rotation, the International Earth Rotation and Reference Systems Service (IERS) announces leap seconds. When one is added, the last minute of 30 June or 31 December has 61 seconds, and clocks read 23:59:60 before midnight. Since 1972, 27 leap seconds have been added, all of them positive. The most recent was at the end of 31 December 2016. That makes UTC exactly 37 seconds behind TAI.
Leap seconds cause real problems for computer systems, which mostly assume every minute has 60 seconds. Software has crashed during leap seconds, and many large operators "smear" the extra second over several hours instead. In November 2022 the General Conference on Weights and Measures decided that, by or before 2035, the permitted difference between UTC and UT1 will be increased, so that leap seconds can stop. The details of how large the new tolerance will be are still being worked out. In recent years the Earth has been rotating slightly faster than before, which has raised the unprecedented possibility of a negative leap second before the change takes effect. None has been needed so far.
GMT itself is no longer measured separately. In practice, "GMT" as a clock time is simply UTC+0, and it follows UTC through any leap seconds.
Side-by-side comparison
| GMT | UTC | |
|---|---|---|
| What it is | A time zone (UTC+0) and a historical astronomical time scale | The international reference time scale |
| Based on | Originally mean solar time at Greenwich | Atomic clocks, kept close to the Earth's rotation |
| Changes for summer? | GMT itself does not, but places that use it in winter may switch to another zone in summer (the UK switches to BST) | Never |
| Who uses it | The UK and Ireland in winter; Iceland and several West African countries all year | Science, aviation, computing, and as the reference for every time zone |
| Legal status in the UK | Under the Interpretation Act 1978, references to time in UK law mean GMT unless stated otherwise (with summer time applied by the Summer Time Act 1972) | Used in technical and international contexts |
The prime meridian has moved, slightly
If you stand on the brass line at the Royal Observatory with a phone, its GPS will tell you that you are not at 0° longitude. The modern reference meridian, used by GPS and maintained by the IERS, lies about 100 metres east of the historic Airy line. This is because modern positioning is based on the Earth's centre of mass, not on the local direction of gravity that the Victorian astronomers used. The time zones do not change: UTC+0 is still UTC+0.
Related time scales you may meet
- Zulu time: the military and aviation name for UTC, from the letter Z used for the zero-offset zone. A time written 1330Z is 13:30 UTC.
- GPS time: the time scale used by satellite navigation. It does not apply leap seconds and has been 18 seconds ahead of UTC since 2017. Receivers convert it for display.
- Unix time: the number of seconds since 00:00 UTC on 1 January 1970, as used by computers. It ignores leap seconds, so each day is counted as exactly 86,400 seconds.
Which should you write?
- For anything technical, such as logs, databases, APIs or flight plans: UTC, ideally in ISO 8601 form, for example
2026-10-17T13:30Z. - For an international audience: UTC, or the local time plus its offset, for example "14:30 London time (UTC+1)".
- For UK local time: write "UK time" rather than "GMT", unless you mean winter only. "GMT" in a July invitation is either wrong or an hour out.
To see the current UTC time, visit our UTC page. For UK time, with the correct GMT or BST label, see London time. Our BST vs GMT guide explains the UK's seasonal switch.