UTC and GMT: the same time, for different reasons

By Niraj Kumar · Published October 28, 2026

For nearly every purpose, Coordinated Universal Time (UTC) and Greenwich Mean Time (GMT) show the same clock reading, and people use the names interchangeably. The difference is in what each one is. GMT is a time zone, a mean solar time, defined by the rotation of the Earth as seen from Greenwich. UTC is a time standard, defined by atomic clocks and adjusted now and then to stay within a second of the Earth. The UTC clock shows the current standard; this guide explains why the standard exists and why the older name survives.

Where GMT came from

Greenwich Mean Time began as the time kept at the Royal Observatory for navigators, who needed a reference meridian to compute longitude. Britain’s railways adopted it nationally in the 1840s to replace local times that differed by minutes between towns, and the 1884 International Meridian Conference chose Greenwich as the world’s prime meridian. “Mean” refers to mean solar time, the average over the year of the sun’s position, because the actual solar day varies by up to half a minute in length. Until the 1920s a GMT day began at noon, the astronomer’s convention; the civil version starting at midnight was for a time called UT to avoid confusion.

Why UTC replaced it

The Earth is not a reliable clock. Its rotation slows unevenly, and by the 1950s atomic clocks were far more stable than the planet. International Atomic Time (TAI), a weighted average of several hundred atomic clocks, became the underlying reference in 1958. But civil time still has to track the sun, or noon would eventually drift into the night. UTC is the compromise: it ticks at the atomic rate, and whenever it drifts more than 0.9 seconds from the Earth’s rotation (UT1), a leap second is inserted to pull it back. It was adopted in 1960 and took its present form, with leap seconds, in 1972.

Even the name was a compromise. English speakers wanted CUT and French speakers TUC; UTC matched neither language and was chosen for that reason, in keeping with UT0, UT1 and UT2.

Leap seconds

Twenty-seven leap seconds have been added since 1972, the most recent at the end of 2016, each one making a UTC day 86,401 seconds long and producing the clock reading 23:59:60. None has ever been removed, though the standard allows it. They are announced about six months in advance by the IERS in Paris, which is why no program can know in advance when the next one will be. In 2022 the General Conference on Weights and Measures resolved to stop inserting them by 2035 and let UTC drift from the Earth by up to a minute or so over the following century; the details are still being settled. UTC today is 37 seconds behind TAI, and GMT, as mean solar time, is within 0.9 seconds of UTC by construction.

Where the difference bites

In practice there are two situations where the distinction matters. The first is precision timing: satellite navigation, telescopes, financial timestamps and network protocols, which need to know that UTC has leap seconds and TAI and GPS time do not. GPS time has not been adjusted since 1980 and is 18 seconds ahead of UTC. The second, far more common, is the United Kingdom in summer. From the last Sunday of March to the last Sunday of October the UK is on British Summer Time, UTC+1. A British clock in July does not show GMT. Writing “3 p.m. GMT” for a London meeting in July is wrong by an hour, and it is a frequent error in international calendar invitations. If you mean the London clock, write London time or BST; if you mean the standard, write UTC.

What to write in software and documents

Use UTC. It is the name of the standard every operating system keeps, the basis of every Unix timestamp, and the Z in an ISO 8601 timestamp. GMT survives in the time zone list as Etc/GMT and in a handful of zones that use it as a label, and some older APIs and HTTP date headers say GMT when they mean UTC; treat those as UTC. The one place GMT is still the correct term is as the name of the UK’s winter time zone, alongside Western European Time, which is the same thing.

A note on the sign of Etc/GMT zones

The IANA database contains zones named Etc/GMT+5 and Etc/GMT-5, and their signs are the reverse of what most people expect: Etc/GMT+5 is five hours behind UTC. The convention comes from POSIX, which expressed offsets as the amount to add to local time to reach UTC. These zones exist for compatibility and should not be used for real places; a city has a name in the database, and that name carries its daylight saving rules, which an Etc zone does not.

Is my clock on UTC

Your device keeps its own count and sets it from a network time server every so often. How far it has drifted since then is something you can measure: the clock check compares the device against a reference and reports the difference. Nothing on this site claims better accuracy than your own clock, and converting between zones with the time zone converter is exact in the offsets but only as right as the clock that feeds it.