The short answer
For everyday purposes, UTC and GMT show the same time. If it is 12:00 UTC, it is also 12:00 GMT. The difference is in the definition:
- GMT (Greenwich Mean Time) is a time zone and an old astronomical time standard, based on the mean solar time at the Royal Observatory in Greenwich, London.
- UTC (Coordinated Universal Time) is the modern international time standard, based on atomic clocks and kept within 0.9 seconds of time based on the Earth’s rotation.
So GMT is best thought of as a time zone used by some countries, and UTC as the reference that all time zones, including GMT, are measured from.
Where GMT came from
In the 19th century, ships navigated using time at a reference meridian, and many nautical charts used Greenwich. As railways and telegraphs spread, countries needed a common reference. At the International Meridian Conference in Washington, D.C., in 1884, delegates chose the meridian through Greenwich as the prime meridian, longitude 0°.
Greenwich Mean Time was the average solar time on that meridian. “Mean” refers to the average: real solar noon drifts by up to about 16 minutes over the year, so astronomers used an averaged sun to keep days of equal length. For decades GMT served as the world’s time reference.
Why UTC replaced it
By the mid-20th century, atomic clocks had shown that the Earth’s rotation is not perfectly steady. It varies slightly from day to day and is slowly getting longer over the long term because of tidal friction. A time scale tied to the Earth’s spin is therefore not uniform enough for science, navigation satellites or telecommunications.
UTC was the solution. It ticks at the rate of International Atomic Time (TAI), which is computed from hundreds of atomic clocks around the world. To keep UTC close to the Earth’s rotation, a leap second can be inserted when the difference grows too large. UTC in its current form, with whole-second steps, has been in use since 1972.
Side-by-side comparison
| GMT | UTC | |
|---|---|---|
| What it is | A time zone, and historically a time standard | The international time standard |
| Based on | Mean solar time at Greenwich | Atomic clocks, adjusted by leap seconds |
| Precision | Limited by the Earth’s uneven rotation | Atomic-clock precision |
| Changes for DST | The UK leaves GMT for BST in summer | Never changes |
| Used by | UK, Ireland, Iceland, parts of West Africa in winter or all year | Science, aviation, computing, the internet |
When the difference matters
For booking a call or reading a timetable, GMT and UTC are interchangeable. The difference matters when:
- Precision is critical. Scientific, satellite and network systems use UTC, which is defined to a tiny fraction of a second. GMT as an astronomical scale can differ from UTC by up to 0.9 seconds.
- Daylight saving time is involved. Saying “London time” or “UK time” is not the same as GMT in summer, when the UK is on BST, one hour ahead. UTC never changes.
- Writing time zone offsets. Standards and software write offsets relative to UTC, such as UTC+5:30 for India. GMT+5:30 means the same thing but is an older style.
Which should you use?
Use UTC when you write timestamps, configure servers, schedule international events or describe offsets. Use GMT when you mean the winter time zone of the United Kingdom or the countries that use it all year. When in doubt, UTC is the safer choice because it is never affected by seasonal clock changes.
A note on the future
In 2022 the General Conference on Weights and Measures decided that leap seconds should be phased out by 2035, allowing a larger gap between UTC and the Earth’s rotation. Even then, UTC and GMT will continue to show the same clock time for all practical purposes.
You can see the current time on the GMT page, compare cities on the world clock or browse all time zones.