UTC vs GMT: what is the difference?
For everyday clocks the two agree to within a second, so people use the names interchangeably. They are measured in completely different ways, and only one of them is a time zone.
GMT is solar time at Greenwich
Greenwich Mean Time is the mean solar time at the Royal Observatory in Greenwich, London. "Mean" matters: the real sun runs fast or slow through the year by up to about sixteen minutes, so GMT averages the length of a solar day rather than tracking the sun exactly.
The 1884 International Meridian Conference in Washington, D.C. recommended the Greenwich meridian as the world reference, which made GMT the practical basis for international timekeeping for most of the next century.
Confusingly, GMT is also still the name of a civil time zone. The United Kingdom uses GMT in winter and British Summer Time in the summer, so "GMT" can mean either a global reference or the UK clock, depending on who is talking.
UTC is an atomic time scale
Coordinated Universal Time was developed in the 1960s and formally adopted in 1972. It counts seconds using atomic clocks rather than the rotation of the Earth, which makes it far more stable than any astronomical measure.
Because the Earth does not rotate at a perfectly constant rate, UTC would slowly drift away from solar time if left alone. Leap seconds were introduced to keep it within 0.9 seconds of UT1, the modern precise version of solar time. In 2022 the General Conference on Weights and Measures agreed to stop inserting leap seconds by 2035.
UTC never observes daylight saving. It has no summer variant and no regional exceptions, which is exactly why it works as the anchor that every regional offset is expressed against.
Which one should you use?
Use UTC. Aviation, shipping, science, and effectively all computer systems standardise on it, and every IANA time zone is defined as an offset from UTC rather than from GMT.
Write offsets in the UTC±HH:MM form — UTC+09:00, UTC−05:00 — rather than as a bare abbreviation. Offsets are unambiguous; abbreviations are not.
Reserve GMT for the UK winter clock or for historical context. If you mean the global reference, say UTC and avoid the ambiguity entirely.
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Frequently asked questions
- Are UTC and GMT ever different?
- For civil purposes they are the same to within a second and you can treat them as equal. They differ in how they are defined and measured: UTC is derived from atomic clocks, GMT from the average solar day at Greenwich. Only precision timing work needs to care.
- Is UTC a time zone?
- No. UTC is the reference scale that time zones are defined against. A time zone is a region plus a rule for its offset from UTC, including any daylight saving changes.
- Why is it abbreviated UTC and not CUT?
- It is a compromise. English speakers wanted CUT for "coordinated universal time" and French speakers wanted TUC for "temps universel coordonné". UTC matches neither language, so nobody was favoured.
- What happens when leap seconds are discontinued?
- UTC will be allowed to drift further from solar time before any correction is applied. The change is scheduled to take effect by 2035, and the main benefit is removing a recurring source of failures in computer systems that cannot represent a 61-second minute.