UTC, GMT and Zulu: What the Difference Actually Is

GMT started as astronomy and became a time zone

Greenwich Mean Time began as a measurement of the Earth's rotation: mean solar time at the Royal Observatory in Greenwich. It was a scientific time scale, and for most of the nineteenth and twentieth centuries it was the world's reference.

That original meaning has been retired. The modern successor to astronomical GMT is called UT1, and it still tracks the actual rotation of the Earth, which turns out to be slightly irregular. The planet speeds up and slows down by small amounts, so a time scale based on it does not tick evenly.

What survives as "GMT" in ordinary speech is something else entirely: a time zone, equal to UTC+0, used by the United Kingdom in winter and by several West African countries year round.

UTC is atomic, with corrections

Coordinated Universal Time is built on atomic clocks, which tick at a genuinely constant rate, unlike the Earth. That makes it suitable as a reference for science, navigation and computing.

But a purely atomic scale would slowly drift away from the sun, and eventually noon would not be noon. So UTC is kept within 0.9 seconds of UT1 by occasionally inserting a leap second. Twenty-seven have been added since 1972, every one of them positive. The most recent was at the end of 2016.

Leap seconds cannot be predicted far ahead, because the Earth's rotation cannot be predicted far ahead. They are announced only months in advance, which makes them awkward for computer systems. Interestingly, the planet has been rotating slightly faster in recent years, raising the previously theoretical possibility of a negative leap second, which no software has ever had to handle.

In 2022 the international body responsible for measurement standards resolved to abolish leap seconds by 2035, letting UTC and the Earth's rotation drift apart and dealing with the gap some other way.

The name is a compromise, which is why it matches neither English nor French word order. English would give CUT, French would give TUC, so the committee settled on UTC and made it the same in every language.

Zulu is just the letter Z

In the ISO 8601 timestamp format, a trailing Z marks a time as having zero offset from UTC: 2026-10-18T14:30:00Z. In the NATO phonetic alphabet the letter Z is spoken "Zulu", so aviation and military usage says "fourteen thirty Zulu".

That is the whole story. Zulu time is UTC. There is no technical difference, only a different way of saying it, adopted because radio communication needs unambiguous spoken letters.

The mistake almost everyone in Britain makes

Here is where the distinction stops being academic.

The United Kingdom is on GMT in winter. From late March to late October it is on British Summer Time, which is UTC+1. People nonetheless say "GMT" all year, and write "9am GMT" in June, when what they actually mean is 9am BST, an hour earlier in absolute terms.

For half the year, "GMT" used to mean "UK time" is simply wrong, and it is wrong by exactly the amount that causes someone to join a call an hour late. The London page shows which one is currently in effect rather than assuming.

Which to use, and when

• Use UTC for anything technical: timestamps, logs, APIs, specifications, coordinating across regions. It is unambiguous and never changes with the season.
• Avoid GMT in writing unless you specifically mean the UK in winter or a West African country. If you mean zero offset, write UTC.
• Use Zulu only in contexts that already expect it, which in practice means aviation, maritime and military work.
• Use neither for scheduling meetings. This is the one people get wrong in the other direction. A meeting belongs in a named zone such as Europe/London or America/New_York, not in an offset, because the offset of a place changes through the year while the name does not.

That last point is the practical heart of it. UTC is the right way to record a moment. A named time zone is the right way to schedule a future event. Confusing the two is the single most common source of calendar errors across borders.

Related reading

• Why your recurring meeting shows the wrong time twice a year
• Where named time zones like Europe/London actually come from