GMT vs UTC: What Is the Difference?
For everyday purposes GMT and UTC show the same time. The difference is what they are: UTC (Coordinated Universal Time) is the global time standard kept by atomic clocks, while GMT (Greenwich Mean Time) is a time zone used by the UK and some other countries in winter.
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The short version
| GMT | UTC | |
|---|---|---|
| What it is | A time zone | A time standard |
| Based on | Originally the Sun at Greenwich, London | Atomic clocks, adjusted to track the Earth |
| Offset | UTC+0 | The zero reference itself |
| Daylight saving | The UK leaves GMT for BST (UTC+1) in summer | Never changes |
| Used by | UK, Ireland, Portugal and parts of West Africa (some only in winter) | Aviation, computing, science, the internet |
Where GMT came from
Greenwich Mean Time began as the average solar time at the Royal Observatory in Greenwich, London. In the 19th century it gave British railways a single time, and in 1884 an international conference chose the Greenwich meridian as the world's zero line of longitude. For decades GMT was the reference that every other time zone was measured from.
The problem is that the Earth is not a perfect clock. Its rotation wobbles and slows very slightly, so time based on the Sun drifts by tiny amounts. That was fine for railways but not for satellites, radio and computer networks.
Why UTC replaced it
UTC was introduced in the 1960s and took its current form in 1972. It is kept by hundreds of atomic clocks around the world, which are vastly more stable than the Earth's rotation. To stop it drifting away from the Sun, UTC was occasionally adjusted with a leap second, keeping it within 0.9 seconds of solar time.
So today, UTC is the standard and GMT is a zone defined against it as UTC+0. A clock set to GMT and a clock set to UTC will always show the same time, but technically correct writing uses UTC for anything precise, such as timestamps, flight plans and scientific data.
The summer confusion: GMT, BST and London time
The most common mistake is assuming London is always on GMT. It is not. The UK uses GMT from late October to late March and British Summer Time (BST, UTC+1) for the rest of the year. If someone in summer says a meeting is at "3 pm GMT", they very often mean 3 pm London time, which is actually 2 pm UTC.
When you schedule across countries, say "UTC" or name the city ("3 pm London time") rather than "GMT". The time zone converter and the meeting planner both handle daylight saving automatically.
Reading UTC offsets
Every time zone is described as UTC plus or minus a number of hours. A few common ones:
- New York: UTC−5 in winter (EST), UTC−4 in summer (EDT)
- Los Angeles: UTC−8 in winter (PST), UTC−7 in summer (PDT)
- Central Europe: UTC+1 in winter (CET), UTC+2 in summer (CEST)
- India: UTC+5:30 all year (IST), with no daylight saving
- Japan: UTC+9 all year (JST)
- Sydney: UTC+10 (AEST), UTC+11 in the southern summer (AEDT)
To convert, start from UTC and add the offset. 14:00 UTC is 09:00 in New York in winter (14 − 5) and 19:30 in India (14 + 5:30). The world clock shows the current offset for each city you pin, so you never have to remember which half of the year it is.
Which countries use GMT?
Several countries keep their clocks on UTC+0 and call it GMT, but not all of them stay there all year.
| Country or territory | Winter | Summer |
|---|---|---|
| United Kingdom | GMT (UTC+0) | BST (UTC+1) |
| Ireland | GMT (UTC+0) | IST, Irish Standard Time (UTC+1) |
| Portugal (mainland) | WET (UTC+0) | WEST (UTC+1) |
| Iceland | GMT (UTC+0) | GMT (no change) |
| Ghana, Senegal, Ivory Coast and other West African states | GMT (UTC+0) | GMT (no change) |
So "GMT" on a clock in Accra or Reykjavik means the same thing all year, while in London or Dublin it only applies from late October to late March.
Writing times so nobody gets confused
- For international audiences, give UTC plus one local reference: "14:00 UTC (15:00 London, 10:00 New York)".
- Use 24-hour time in technical writing, logs and schedules. It removes am/pm mix-ups.
- In software, store times in UTC and convert to the viewer's zone only for display. ISO 8601 format, such as 2026-10-09T14:00:00Z, is unambiguous; the Z means UTC.
- Avoid three-letter abbreviations where you can. IST means India, Irish or Israel Standard Time depending on who is reading, and CST covers both US Central and China Standard Time.
- When daylight saving matters, name the city, not the zone. "9 am New York time" is correct all year; "9 am EST" is wrong for half of it.
If you work with timestamps in code, our guide to Unix time explains why storing seconds since 1970 in UTC avoids most of these problems.
Zulu time and the military
Pilots, the military and weather services call UTC "Zulu time", from the NATO phonetic letter Z for the zero zone. A flight briefing that says 1400Z means 14:00 UTC. These times are always written in 24-hour format, which the military time converter translates into ordinary 12-hour time.
Frequently asked questions
Is GMT the same as UTC?
They show the same clock time, but they are not the same thing. UTC is the atomic-clock time standard; GMT is a time zone at UTC+0.
Does UTC change for daylight saving time?
No. UTC never changes. Countries move their own offset from UTC instead, such as the UK switching from GMT (UTC+0) to BST (UTC+1).
Is London time always GMT?
No. London is on GMT in winter and British Summer Time (UTC+1) from late March to late October.
What is Zulu time?
Zulu time is another name for UTC, used in aviation and the military. 0900Z means 09:00 UTC.
Should I use GMT or UTC?
Use UTC for anything technical or international, such as software, logs and scheduling. Use GMT only when you mean the UK winter time zone.
What is the time difference between GMT and UTC?
There is no difference in clock time. Both are at UTC+0. The only gap is the sub-second difference between atomic time and solar time, which does not affect clocks.
Is GMT ahead of UTC?
No. GMT is defined as UTC+0, so they show the same time.
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