GMT vs UTC: What's the Difference?
GMT is a time zone, UTC is a time standard — the precise difference, why aviation and computing use UTC, and what Zulu time means.
GMT and UTC show the same clock reading, but they are different kinds of things: GMT is a time zone; UTC is a time standard. GMT — Greenwich Mean Time — refers to mean solar time at the Royal Observatory in London. UTC — Coordinated Universal Time — is the precision timekeeping backbone that the world's clocks agree to follow.
The core differences
| GMT | UTC | |
|---|---|---|
| What it is | A time zone (mean solar time at Greenwich) | A time standard maintained by atomic clocks |
| Basis | Earth's rotation relative to the sun | SI seconds from atomic clocks, corrected by leap seconds |
| Used as | Civil time in UK, Ireland, Portugal (winter) | Global reference in aviation, computing, science, GPS |
| Precision | Astronomical — accurate to the second at best | Atomic — nanosecond scale |
Why UTC needs leap seconds
Atomic seconds are constant, but Earth's rotation is slowly braking — the day lengthens by about 1.7 milliseconds per century. Left alone, atomic time would drift away from the sun. Since 1972, leap seconds have occasionally been inserted into UTC to keep it within 0.9 s of astronomical time. There have been 27 so far; the last was added at the end of 2016, and in 2022 the General Conference on Weights and Measures voted to retire the leap second by 2035.
Zulu, Z, and the offset system
- In aviation and the military, UTC is called "Zulu time" and written with a Z suffix: 14:30Z
- Timestamps like 2026-10-04T14:30:00Z in logs and APIs use the same Z
- Every offset is expressed relative to UTC: New York is UTC−5 in winter, Tokyo is UTC+9 year-round, Kathmandu famously UTC+5:45
One practical wrinkle: London is on GMT only in winter. In summer the UK switches to BST (UTC+1), so "GMT" as a live clock label is wrong for half the year — our London page shows the actual current offset, whatever the season.
Frequently asked questions
Is GMT the same as UTC?
They show the same time, but GMT is a time zone based on solar time at Greenwich, while UTC is an atomic-clock time standard. In practice UTC is the successor used everywhere precision matters.
Why is it called Zulu time?
The military assigns letter codes to offsets; Z (zulu) denotes UTC+0. Aviation adopted it, so 14:30Z means 14:30 UTC.
Does UTC have daylight saving time?
No. UTC never shifts. Countries change their offset relative to UTC — for example the UK goes from UTC+0 (GMT) to UTC+1 (BST) in summer.