What is UTC time?
The reference every other zone is defined against — atomic, unchanging, and the only sensible thing to write down when a timestamp has to survive being read elsewhere.
The short answer
UTC — Coordinated Universal Time — is the reference all civil time zones are defined against. Every zone is UTC plus or minus an offset. It is — right now.
It never changes: no daylight saving, no seasonal adjustment, no regional variation. That is what makes it useful.
What it is made of
UTC is kept by a weighted average of some four hundred atomic clocks in laboratories around the world, coordinated by the International Bureau of Weights and Measures.
Atomic time is extremely stable. The earth's rotation is not — it varies slightly and unpredictably. UTC is deliberately steered to stay within a second of solar time, which is what makes it usable as civil time rather than merely as a physics constant.
Leap seconds
When atomic time and the earth's rotation drift nearly a second apart, a leap second is inserted — a real 23:59:60 — to bring them back in line.
These have been added irregularly since 1972. They cause genuine trouble in computing, because a minute with 61 seconds breaks software that assumes otherwise, and several large outages have been traced to them. The international body responsible has voted to discontinue the practice by 2035.
UTC versus GMT
For everyday purposes: interchangeable, both UTC+0.
Strictly they are different kinds of thing. UTC is a time standard defined by atomic clocks. GMT is a time zone, based on mean solar time at the Greenwich meridian, and it is the UK's winter clock — in summer the UK is on UTC+1.
So "GMT" carries a national and seasonal meaning that UTC does not. When writing something down, UTC is the better term for exactly that reason.
Why the abbreviation does not match the name
"Coordinated Universal Time" would be CUT in English and TUC in French. Neither side would accept the other's, so the compromise was UTC, which matches neither language and follows the pattern of other ITU time designations (UT0, UT1, UT2).
Where you will meet it
Aviation and shipping. Flight plans, air traffic control and crew duty times all run on UTC, because an aircraft crossing six zones cannot use local time for anything operational.
Software and logs. Well-built systems store timestamps in UTC and convert to local time only for display. It is the single most effective defence against daylight-saving bugs.
Anything scientific. Observations, telemetry and experiments are timestamped in UTC so results from different countries can be compared without ambiguity.
Writing a UTC timestamp
The ISO 8601 form is 2026-07-15T14:30:00Z. The trailing Z means UTC.
That string means one unambiguous instant everywhere on earth, which is more than can be said for "3pm Thursday."
Related: UTC to EST · What does GMT mean? · UTC offsets: the full list
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Figures on this page are computed from the IANA time zone database. Last generated 2026-08-18.