Time zones and clocks
UTC vs GMT: what the difference is, and when it matters
For most everyday purposes UTC and GMT give the same clock reading. They are still different things, and the difference explains leap seconds, the letter Z, and why Britain is not "on GMT" in summer.
Last reviewed October 5, 2026About 3 min to readSite version v2026.10.1
GMT: mean solar time at one observatory
Greenwich Mean Time is the mean solar time on the meridian running through the Royal Observatory at Greenwich, London. "Mean" matters: the real sun runs up to about sixteen minutes fast or slow across the year, for reasons covered in the equation of time guide, so GMT follows an averaged, fictitious sun that moves at a constant rate.
Because it is tied to Earth's rotation, GMT is an astronomical time scale. It was the world's reference time from the 1884 Meridian Conference until the atomic era, and it survives today in two roles: as the name of the UTC+0 time zone used by the United Kingdom, Ireland and Portugal in winter, and by several West African countries all year, and as a loose everyday synonym for UTC.
UTC: atomic seconds, nudged to stay near the sun
Coordinated Universal Time is built from a weighted average of several hundred atomic clocks around the world. Its seconds are defined by the caesium atom, not by Earth's spin, and Earth's spin is slightly irregular: tides slow it, and the core and atmosphere shift its speed by milliseconds a day. Left alone, atomic time would drift away from the sun.
To prevent that, UTC is kept within 0.9 seconds of the rotation-based scale UT1 by occasionally inserting a leap second. Twenty-seven have been added since the system began in 1972, the most recent at the end of December 2016. In 2022 the General Conference on Weights and Measures resolved to stop adding leap seconds by 2035, so the two scales will be allowed to drift apart slowly from then on. The decision belongs to that body and to the International Telecommunication Union, and it can change.
The abbreviation itself is a diplomatic compromise. English speakers wanted CUT, French speakers wanted TUC, and the committee settled on UTC, which matches neither language and offends nobody.
So are they the same?
| GMT | UTC | |
|---|---|---|
| What it is | A time zone, and historically a solar time scale | A time standard defined by atomic clocks |
| Defined by | Mean solar time at Greenwich | International Atomic Time minus leap seconds |
| Offset | UTC+0 (as a zone) | The zero point every offset is measured from |
| Precision | Everyday use | Scientific, aviation, computing, navigation |
| Observes daylight saving? | No; the UK switches to BST, a different zone | Never |
In practice the clock readings never differ by more than a fraction of a second, so for scheduling a call or reading a timetable the two are interchangeable. In writing, prefer UTC. It is unambiguous, it is what aviation, shipping, computing and every standards body use, and it never carries the "but is it summer time?" question that GMT does.
Why Britain is not on GMT in summer
The commonest confusion is to treat GMT as "London time". From the last Sunday in March to the last Sunday in October the United Kingdom is on British Summer Time, UTC+1. A meeting booked for "2 PM GMT" in July is at 3 PM on a London clock. The United Kingdom page shows the live clock and both changeover dates; the BST abbreviation page explains the code.
The same trap exists for any country that names its winter zone after a reference time. Writing "14:00 UTC" or "2 PM London time" avoids it; writing "2 PM GMT" in summer invites it.
Z, Zulu and ISO 8601
In timestamps the letter Z marks UTC: 2026-07-04T14:00:00Z is 2 PM UTC on July 4, 2026, written in the ISO 8601 format most software exchanges. Aviation and the military read the Z aloud as "Zulu", so "1400 Zulu" is the same instant. Writing the offset explicitly, as in 2026-07-04T10:00:00-04:00, says the same thing from New York's point of view.
The converter on the home page accepts UTC as a clock in its own right. Add it to any comparison and you can read the shared instant in Z form next to each local time, which is the cleanest way to put a meeting in writing for people in several countries.
See it on the site
- UTC explainedThe abbreviation page, with live clocks for the places that keep UTC+0 all year.
- GMT explainedWhat the GMT code means on each side of the British clock change.
- GMT to EST chartAn hour-by-hour conversion table with dated examples.