UTC vs GMT: what's the difference?

If you have ever seen a meeting invite that says "3pm UTC" and another that says "3pm GMT" and wondered whether they are the same moment, the answer is yes, they are. The confusion is real, though, and worth clearing up, because only one of these is safe to rely on when you are coordinating people in different countries.

What is GMT?

Greenwich Mean Time is a time zone. It is the local time at the Royal Observatory in Greenwich, London, and historically it was defined by the position of the sun over that meridian. Several countries set their clocks to GMT (UTC+0), including the UK in winter, Iceland, and parts of West Africa.

Because it is a civil time zone, GMT is something a country can adopt, leave, and rejoin. That is exactly what the UK does: it is on GMT in winter and switches to British Summer Time (BST, UTC+1) from late March to late October. So "GMT" does not reliably mean "the time in London."

What is UTC?

Coordinated Universal Time is a time standard, not a zone. It is maintained by a global network of atomic clocks and is the reference that every other time zone is defined against. New York is "UTC-5", Mumbai is "UTC+5:30", Tokyo is "UTC+9", and so on. UTC itself is nobody's permanent local civil time; it is the ruler everyone measures from.

Crucially, UTC never shifts for daylight saving. It is fixed, precise, and unambiguous, which is why servers, databases, aviation, and scheduling systems all run on it.

Are they the same time?

For everyday use, yes. UTC and GMT both sit at +0, so a clock showing GMT and a clock showing UTC read the same. The technical difference is tiny: UTC is kept by atomic clocks and is allowed to drift up to 0.9 seconds from astronomical (sun-based) time before a "leap second" is inserted to bring it back in line. GMT, in its original astronomical sense, follows the sun directly. You will never notice this difference in a meeting.

Which should you use for scheduling?

Use UTC. When you write "the call is at 15:00 UTC", there is exactly one moment that refers to, and it does not move when a clock changes somewhere. If you write "15:00 GMT" in July, a careful reader will pause: is that really UTC+0, or did you mean current UK time, which is BST (UTC+1) in July? That one-hour ambiguity is precisely the kind of mistake that makes someone miss a meeting.

A quick history (and why it changed)

GMT was the world's time standard from 1884, when the Greenwich meridian was adopted as the prime meridian. As atomic clocks became far more accurate than the Earth's slightly irregular rotation, the world needed a standard based on something steadier. UTC was introduced in 1960 and formally replaced GMT as the international reference in 1972. GMT lived on as a civil time zone; UTC took over as the scientific standard.

The bottom line

GMT is a place on the clock that some countries occupy; UTC is the clock everyone reads from. They agree to the second for any practical purpose, but when you are coordinating people in different countries, reach for UTC as your reference and translate to each person's local time when you actually send the invite. If you want to skip the translation entirely, Atlas keeps every teammate's local time one glance away and writes the meeting to your calendar in everyone's correct zone. For the full method, see how to schedule a meeting across time zones.