UTC vs. GMT: what's the actual difference?

In everyday conversation, UTC (Coordinated Universal Time) and GMT (Greenwich Mean Time) are treated as the same thing, and for most practical purposes — like reading a flight schedule — they are. But they come from different systems and aren't technically identical.

GMT is a timezone

GMT is the local mean solar time at the Royal Observatory in Greenwich, London, and it's a specific timezone — the one the UK uses in winter. It's tied to the Earth's rotation, which isn't perfectly constant.

UTC is a time standard

UTC is not a timezone at all — it's the reference standard that every timezone in the world is defined as an offset from (e.g. "UTC+9" for Tokyo, "UTC-5" for New York in winter). It's based on atomic clocks rather than the Earth's rotation, with occasional "leap seconds" inserted to keep it aligned with solar time. This is the standard used in aviation, computing, scientific timestamps, and — behind the scenes — this converter.

So when are they different?

In practice, UTC and GMT are within a fraction of a second of each other at any given moment, which is why they're used interchangeably in casual contexts. The more meaningful difference people run into is that GMT, as a UK timezone, shifts to BST (British Summer Time, UTC+1) during daylight saving — so "London is on GMT" is only true for part of the year. UTC itself never observes daylight saving; it's a fixed reference point year-round.

Which one should you use?

For international meeting invites, flight itineraries, or server timestamps, UTC is the safer label because it's unambiguous and doesn't shift with the seasons. "3pm GMT" is technically ambiguous for half the year in the UK itself. If you're specifying a UK local time, it's more precise to say "London time" and let a converter handle whether that's GMT or BST for that date.