On a clock they match; as definitions they differ#
If you only need to know what time it is, UTC and GMT point to the same moment. Both are written as an offset of zero, UTC+00:00, and for ordinary purposes they agree to within a second.
The difference is in what each name means. GMT is a Sun-based time that became the name of a clock people live by. UTC is an atomic time scale with no seasonal changes. The trap is that clock: Britain leaves GMT every spring, while UTC never moves.
What GMT is, and why the UK is on it only in winter#
According to Royal Museums Greenwich, Greenwich Mean Time is the yearly average of the time each day when the Sun crosses the Prime Meridian at the Royal Observatory in Greenwich. "Mean" means average: the real Sun runs a little fast or slow through the year, so an average day gives a steady standard. The museum says GMT was the international civil time standard from 1884 to 1972, when UTC replaced it, and that it remains the legal winter time in Britain. The earlier story is in How Time Zones Were Born: Railways, Telegraphs and the Day of Two Noons.
Summer is where the confusion starts. The Summer Time Act 1972 says that during summer time, the time for general purposes in Great Britain is one hour in advance of Greenwich mean time. The Act leaves the period to be set by an Order, and the Summer Time Order 2002 sets it: summer time begins at 1 a.m. GMT on the last Sunday in March and ends at 1 a.m. GMT on the last Sunday in October. The summer clock is British Summer Time (BST), UTC+01:00.
The dates, computed from the IANA time zone database for London:
- 2026: clocks go forward on Sunday, March 29 and back on Sunday, October 25. BST lasts 210 days.
- 2027: clocks go forward on Sunday, March 28 and back on Sunday, October 31.
So on October 9, 2026, London is on BST. It returns to GMT on October 25 and stays there for 154 days, until March 28, 2027 (rules as of October 2026). When Do the Clocks Change? Daylight Saving Dates for 2026 and 2027 has the dates for other regions and How Daylight Saving Time Works, Why It Exists and Who Still Uses It explains the mechanism.
What UTC is, and why it is not called CUT#
Coordinated Universal Time takes its seconds from atomic clocks. Under ITU-R Recommendation TF.460-6, UTC differs from International Atomic Time by a whole number of seconds, chosen so that UTC stays within 0.9 seconds of UT1, which is time defined by Earth's slightly irregular rotation. When the gap threatens to reach that limit, the International Earth Rotation and Reference Systems Service (IERS) announces a leap second. How Your Phone Knows the Time: Atomic Clocks, Leap Seconds and the Network Behind Them covers the clocks and networks behind it.
Why the abbreviation is UTC#
The English name would shorten to CUT and the French, temps universel coordonné, to TUC. According to timeanddate.com, the International Telecommunication Union and the International Astronomical Union wanted one abbreviation for every language, so they chose UTC, which favors neither and fits UT0 and UT1.
That is the usual explanation, but it is disputed. In a 2019 post on the IANA time zone mailing list, Steve Allen wrote that there is no contemporary evidence for a compromise. He argued that the name grew out of the Universal Time names adopted in 1955: UT0, UT1 and UT2 in English, TU0, TU1 and TU2 in French. Sources differ on the origin.
When UTC and GMT really differ#
| GMT | UTC | |
|---|---|---|
| What it is | Mean solar time at Greenwich, tied to the Sun and Earth's rotation | A time scale run on atomic clocks |
| Leap seconds | Not part of it | Added when needed to stay within 0.9 s of UT1 |
| Role today | The UK's legal winter time | Replaced GMT as the international standard in 1972 |
| Daylight saving | The UK leaves GMT for BST in summer | None, ever |
The gap between UTC and Earth-rotation time is called DUT1 (UT1 minus UTC), and radio time signals broadcast it in steps of a tenth of a second. It stays under 0.9 seconds, so if GMT is read as Earth-rotation time, the two names differ by less than a second. That matters for astronomy and satellite navigation, not for a meeting.
The difference people actually meet is the daylight saving row: the name GMT is used for a clock that moves in summer, and UTC is not.
GMT, UTC, BST and WET side by side#
Western European Time (WET) is the clock of mainland Portugal, among other places. Like the UK, it moves one hour ahead in summer, to Western European Summer Time (WEST).
| Name | Meaning | Winter | Summer | Where |
|---|---|---|---|---|
| UTC | Coordinated Universal Time | UTC+00:00 | UTC+00:00 | The world reference |
| GMT | Greenwich Mean Time | UTC+00:00 | UTC+00:00 | The UK in winter; Iceland and Ghana all year |
| BST | British Summer Time | not used | UTC+01:00 | The UK, late March to late October |
| WET / WEST | Western European (Summer) Time | UTC+00:00 | UTC+01:00 | Mainland Portugal, for example |
Offsets are from the IANA time zone database for January 15 and July 15, 2026. Lisbon changes at the same moment as London in 2026 and 2027, at 01:00 UTC.
Some software labels London's summer time "GMT+1" (Node.js with the en-US locale does, for example), which hides that the UK has left GMT. IST, CST, PST: Why Time Zone Abbreviations Mean Different Things explains why short labels are ambiguous.
Zulu time and the letter Z#
Zulu time is another name for UTC, from the phonetic-alphabet word for the letter Z. The FAA runs air traffic operations on UTC, and its Order JO 7210.3 says the term "ZULU" may be used to denote it. In RFC 3339, the internet's timestamp format, a Z after a time means a UTC offset of 00:00, "often spoken Zulu" after the ICAO phonetic alphabet.
Written aviation times are four digits on a 24-hour clock followed by a letter for the zone. The FAA's own example for local time is 0205M, M for Mountain. Z marks UTC, so 1400Z is 14:00 UTC.
Mind the date: at 8:30 p.m. Eastern Standard Time (UTC-5) in Boston on November 30, 2026, it is 01:30 UTC on December 1, or 0130Z. The time zone converter shows the date at the other end, and 24-Hour vs 12-Hour Clock: Conversion Chart, 12 AM vs 12 PM and Military Time covers the notation.
What to write in invitations, code and logs#
For people. Name the city and add UTC as an anchor: "Wednesday, July 15, 10:00 London time (09:00 UTC)". Avoid a bare "GMT" between late March and late October.
UTC stays fixed while London and New York move:
| 14:00 UTC on | London | New York | Kolkata |
|---|---|---|---|
| January 15, 2026 | 14:00 GMT | 09:00 EST | 19:30 IST |
| July 15, 2026 | 15:00 BST | 10:00 EDT | 19:30 IST |
Kolkata is at UTC+05:30 in both months. The meeting planner can find an hour that suits every city in a group.
For software. Store and exchange timestamps in UTC, written in ISO 8601 style with a Z, such as 2026-07-15T14:00:00Z. Convert to local time only for display, using an IANA zone name such as Europe/London rather than the label "GMT". The zone name knows when London changes; the label does not. Unix Time, ISO 8601 and UTC: How Computers Store Time explains how computers store time.
For flights, logs and shared timelines. Use UTC or Zulu, so everyone reads one clock. Where the exact time has legal force, check the official source: Timelive is not one.
Frequently asked questions
Is UTC the same as GMT?
For everyday purposes, yes: both read UTC+00:00. The definitions differ. GMT is a solar time, the average time of the Sun's crossing of the meridian at Greenwich. UTC counts atomic seconds and is steered by leap seconds to stay within 0.9 seconds of Earth-rotation time.
The practical catch is that GMT is also the name of the UK's winter clock, which becomes British Summer Time (UTC+01:00) in summer. UTC never changes.
What does UTC stand for?
Coordinated Universal Time. The letters match neither the English word order (CUT) nor the French one, temps universel coordonné (TUC). The usual explanation is that the ITU and the International Astronomical Union chose a compromise that favors no language. At least one researcher disputes that story and traces UTC to the Universal Time names UT0, UT1 and UT2 adopted in 1955.
Is the UK on GMT or BST right now?
It depends on the date. In 2026, British Summer Time (UTC+01:00) runs from Sunday, March 29 to Sunday, October 25. The UK returns to Greenwich Mean Time (UTC+00:00) on October 25, 2026 and moves back to BST on Sunday, March 28, 2027. On October 9, 2026, London is on BST, one hour ahead of UTC.
What is Zulu time?
Zulu time is UTC, named after the phonetic-alphabet word for the letter Z. Aviation uses it so that everyone works from one clock, writing times as four digits and a Z: 1400Z is 14:00 UTC. Computer timestamps such as 2026-07-15T14:00:00Z end in the same letter.
Should I write UTC or GMT in a meeting invitation or in code?
Write UTC for a fixed reference time and for anything a computer reads. In an invitation, add the city, for example "10:00 London time (09:00 UTC)", and avoid a bare "GMT" from late March to late October, when it could mean UTC or the London clock an hour apart. In code, store timestamps in UTC with a Z and display them using an IANA zone name such as Europe/London.
Sources and further reading
- The Royal Observatory Greenwich is the home of Greenwich Mean Time (GMT). But what is GMT and why is it so important? — Royal Museums Greenwich
- Recommendation ITU-R TF.460-6: Standard-frequency and time-signal emissions — International Telecommunication Union
- UTC abbreviation — timeanddate.com
- Why is Etc/UCT not an alias of Etc/UTC? (2019 thread) — IANA time zone mailing list
- Summer Time Act 1972 — legislation.gov.uk
- FAA Order JO 7210.3, Facility Operation and Administration, Chapter 2, Section 4 — Federal Aviation Administration
- Date and Time on the Internet: Timestamps (RFC 3339) — Internet Engineering Task Force