Are GMT and UTC the same thing?

For everyday purposes, yes — both have a UTC offset of +0 and represent time at the Greenwich meridian. The technical difference is that UTC is defined by atomic clocks and adjusted with leap seconds, while GMT is defined by the Earth's rotation.

When does the GMT vs UTC distinction matter?

It matters in scientific and engineering work, in computer systems (which use UTC internally), and in UK civil law (which uses GMT in winter, not UTC). For most other uses the two are interchangeable.

Which should I use in code or documentation?

Use UTC. It is the modern standard and what operating systems, APIs, and protocols expect.

GMT vs UTC — What's the difference?

For most everyday uses the two are interchangeable — both are UTC+0. The differences only matter in precise scientific or legal contexts.

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The short answer

For everyday purposes, GMT and UTC are the same. Both have a UTC offset of +0, both represent time at the Greenwich meridian, and most clocks, calendars, and conversion tools treat them interchangeably. If you’re scheduling a meeting or reading a flight time, “10:00 GMT” and “10:00 UTC” mean the same wall-clock moment.

The difference matters in two situations: precise scientific work, and historical or legal contexts in the UK.

The technical difference

	GMT	UTC
Defined by	The Earth’s rotation at the Greenwich meridian	Atomic clocks worldwide
Adopted	Standardised in 1884 at the International Meridian Conference	Introduced in 1960, refined in 1972
Leap seconds	No formal mechanism	Periodically adjusted to keep UTC within 0.9 seconds of solar time
Variation	Drifts slightly with the irregularities of Earth’s rotation	Stable, defined by physics
Used for	UK civil time (in winter), historical contexts, casual reference	Internet protocols, aviation, science, international coordination

UTC was created precisely because GMT had become an imprecise reference: the Earth’s rotation isn’t perfectly uniform, so a time standard tied to solar observation isn’t quite consistent with the millisecond-accurate timekeeping that modern systems require. UTC fixes this by being defined by atomic clocks, then occasionally nudged by leap seconds to stay close to solar time.

When the distinction matters

Scientific and engineering work — astronomers, satellite operators, and global navigation systems use UTC because it’s the precise standard
Computer systems — operating systems and APIs default to UTC; “GMT” in a server log usually means UTC
UK civil law — the United Kingdom’s official time is GMT in winter (not UTC), and BST in summer
Aviation — pilots and air traffic controllers use UTC, often spoken as “Zulu time”

In practice

If you’re booking a meeting or checking a deadline, GMT and UTC are interchangeable. If you’re writing code, log files, or technical documentation, use UTC. If you’re discussing British civil time in winter, GMT is the correct term.

See the standalone pages for UTC and GMT for current time and conversions to major cities.