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UTC, GMT and Zulu — three names, one clock

UTC, GMT and Zulu all point at essentially the same instant, yet they come from different worlds: metrology, astronomy and the military. For everyday scheduling they are interchangeable. For log files, aviation and legal timestamps, the distinctions start to matter.

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Where each term comes from

GMT is Greenwich Mean Time, defined by the sun's position over the Greenwich meridian. It is an astronomical measure, and it was the world's reference until the middle of the twentieth century.

UTC is Coordinated Universal Time, defined by atomic clocks and kept within 0.9 seconds of astronomical time by the occasional insertion of a leap second. It replaced GMT as the international reference in 1972. In casual use the two are the same; in precise use UTC is the standard and GMT is a time zone that happens to match it in winter.

Zulu is the NATO phonetic letter for Z, the suffix used in the ISO 8601 timestamp format — 2026-08-14T09:30:00Z. Aviation and the military say "zero nine thirty zulu" to make it unambiguous over a radio. It is UTC with a different pronunciation.

The practical trap is that the United Kingdom is not on UTC for half the year. From late March to late October it runs British Summer Time, UTC+1. A server logging in UTC and a colleague in London reading their watch will disagree by an hour all summer, and that has broken more incident timelines than any other single confusion.

Key facts

UTC is the standard

Since 1972 UTC, not GMT, has been the international reference.

GMT is a time zone

It is also the winter offset of the UK, which is why the two get conflated.

Zulu is UTC

The Z suffix in ISO 8601 timestamps means exactly UTC.

The UK is not always on UTC

From late March to late October London is UTC+1.

UTC never shifts

It has no daylight saving, which is why servers and aviation use it.

Leap seconds are being retired

A 2022 decision will stop adding them by 2035.

How to get it right

1

Store timestamps in UTC

Every database, log and API should record the absolute instant.

2

Convert only for display

Apply the user's zone at the last moment, in the interface.

3

Use ISO 8601 in writing

2026-08-14T09:30:00Z leaves no room for interpretation.

4

Never write GMT for UTC in summer

If you mean the reference clock, say UTC.

Questions people ask

For everyday use, yes — they differ by less than a second. Technically UTC is an atomic standard and GMT is an astronomical one that also serves as the UK winter time zone.

Keep reading

Daylight saving explained

Which countries change their clocks, when they do it, and why the same two cities can be five hours apart one week and six the next.

Scheduling across time zones

A practical method for booking meetings across zones: finding the overlap, rotating the pain, and stopping recurring calls from drifting.

Why abbreviations are ambiguous

IST means three different things, CST means three more. Which abbreviations are ambiguous and what to use instead.

Convert UTC to your zone

Pages for UTC to every major zone, with the daylight saving caveat spelled out.

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