UTC vs. GMT: What's the Actual Difference?
For almost every civil clock, GMT and UTC show the same time. The difference is the definition, and it is why a timestamp in a database should say UTC even when a person would have said GMT.
| GMT | UTC | |
|---|---|---|
| What it is | Mean solar time at the Greenwich meridian. Also the name of UK civil time in winter. | The atomic time scale used for timestamps, aviation, and navigation. |
| Basis | The sun’s average motion | International Atomic Time (TAI), plus a whole-second offset |
| Leap seconds | Not part of the solar definition | 27 were inserted from 1972 through 31 December 2016. None since. |
| A time zone? | Yes, when someone means the UK in winter | No. Zones are counted from it. |
| Daylight saving | The UK leaves it for British Summer Time | Never |
“Mean” is doing real work
True solar noon, the moment the sun crosses the meridian, is not a tidy 24 hours later each day. The Earth’s orbit is not a circle and the axis is tilted, so solar noon wanders from clock noon by up to about 16 minutes across the year. Astronomers call that the equation of time. “Mean” time averages the wander into a steady day. Without that average, a civil clock tied to the sun would have to run fast and slow with the seasons.
GMT came from the observatory
Greenwich Mean Time was defined from the sun at the Royal Observatory. The International Meridian Conference of 1884 recommended that meridian as the world’s reference, and railways and telegraph networks needed one shared clock instead of a different solar noon in every town.
GMT follows the Earth’s rotation. The rotation is not a perfect clock. Tidal friction and the movement of mass in the oceans, the air, and the interior speed it up and slow it down. Fine for catching a train in 1884. Not fine for a satellite or a network that needs a second of constant length.
UTC is the atomic scale
Coordinated Universal Time was built in the 1960s and took its current form in 1972. It ticks with International Atomic Time, which is a weighted average of atomic clocks, and then it is held near the older solar scale by inserting or, in theory, removing a whole second.
From 1972 until the end of 2016, that correction was a leap second, applied so that UTC stayed within 0.9 seconds of UT1, the modern measure of Earth rotation. Twenty-seven leap seconds were inserted. All of them were positive. The last one was on 31 December 2016. Since January 2017, TAI has been 37 seconds ahead of UTC: 10 seconds of older offset, plus those 27.
A computer that assumes every minute has 60 seconds breaks on the extra second. That is the operational complaint, and it is a fair one.
What changed in 2022, and again in 2026
In 2022 the General Conference on Weights and Measures passed Resolution 4, telling the timekeeping bodies to raise the allowed gap between UTC and Earth rotation in or before 2035, so that UTC could run without leap seconds for at least a century.
In October 2026 the same conference adopted Resolution 5. Continuous UTC starts on 20 May 2027. The allowed gap, |UT1 minus UTC|, rises from 0.9 seconds to 3600 seconds, one hour. That is enough room that nobody alive now has to insert another leap second to satisfy the rule. It is not a claim that the sun will stay overhead at noon forever. An hour of drift is the ceiling a later generation is allowed to hit before they have to do something. For a meeting, a log line, or a flight plan, the change means the minute you stored still has 60 seconds.
I am not a delegate to that conference. The text of Resolution 5 is a BIPM document. If this page and that document ever disagree, the document wins.
How a laptop gets the time
Phones, laptops, and servers set their clocks from the Network Time Protocol. NTP servers are referenced to UTC. GMT is not in that chain, except that a UK winter clock is defined to show the same hours and minutes. Two machines that both synced recently agree. A machine that has not synced, or a program that parsed “GMT” as a label and then applied a daylight-saving rule, will not.
Which word to use
In speech, “3:00 PM GMT” and “3:00 PM UTC” are the same instant for any purpose that does not care about a fraction of a second. The trap is that GMT is also a time zone name. The UK is on GMT in winter and on British Summer Time, one hour ahead, from the last Sunday of March to the last Sunday of October. Someone who says “GMT” in July may mean the UK’s current civil time, which is not GMT, or they may mean the zero meridian. You cannot tell from the three letters.
UTC does not have a summer. That is why a stored timestamp should use it.
- Say UTC in a schema, a log, and an API. A legislature cannot move it by an hour.
Zin an ISO 8601 timestamp, and “Zulu” on a flight plan (14:00Z), both mean UTC. Aviation picked a clock with no country attached to it.- A legacy system that prints “GMT” on a UTC value is describing the right instant and the wrong name. Do not “fix” it by adding an hour in summer.
Store the instant in UTC. Convert to a place only when a person is looking at it. The bugs that come from storing the local wall time instead are written out in why developers should store UTC, including the hour that does not exist and the hour that happens twice. The homepage does the civil-time conversion from your browser’s copy of the IANA database. It is not a source of legal time, and it is not the BIPM.