UTC vs GMT: What's the Difference?
For everyday purposes they're interchangeable. They differ in what they're measured against — the sun over Greenwich versus a global network of atomic clocks — and in which one you should actually write down.
If you've scheduled a call with London and hesitated over whether to write UTC or GMT, the short answer is that it rarely matters: the two are within a second of each other, and for arranging a meeting they're the same thing. The longer answer is that they're different kinds of thing, and knowing which is which tells you when to use each.
GMT: Solar Time From Greenwich
Greenwich Mean Time is the older of the two. It takes its name from the Royal Observatory at Greenwich in London, which was adopted as the world's prime meridian — longitude zero — at the International Meridian Conference in 1884.
GMT is solar time: it's based on the average interval between one solar noon at Greenwich and the next. That made it an excellent standard for an era of railway timetables and ship navigation, and for most of a century it was the reference the rest of the world set its clocks by.
Its limitation is the Earth itself. The planet's rotation isn't perfectly regular — it varies slightly and is very gradually slowing — so a standard defined by rotation drifts in ways that matter once you can measure time more precisely than the Earth turns.
UTC: Atomic Time, Kept in Step
Coordinated Universal Time was introduced in 1960 and became the official international standard in 1972. It's derived from a worldwide network of several hundred atomic clocks, which measure time by the resonant frequency of caesium atoms rather than by astronomy.
But UTC still has to match the world people live in, where noon should fall near the middle of the day. Since atomic time and the Earth's rotation drift apart, UTC is occasionally corrected by a leap second — an extra second inserted to keep it within 0.9 seconds of astronomical time. Twenty-seven have been added since 1972, and in 2022 international agreement was reached to phase the practice out by 2035, because the disruption leap seconds cause to computer systems now outweighs their benefit.
Incidentally, the abbreviation is a compromise: English speakers proposed CUT, French speakers TUC, and UTC was adopted so that no language got its own initials.
The Practical Differences
- What they measure against. GMT is astronomical, based on the sun's position at Greenwich. UTC is atomic, adjusted to stay close to it
- Precision. UTC is defined to far greater accuracy; GMT inherits the irregularity of the Earth's rotation
- Leap seconds. A UTC concept, used to reconcile the two. GMT has no such mechanism
- Status. UTC is the international standard used in computing, aviation, science and law. GMT today functions mainly as the name of a time zone — the UK's winter time — rather than as a standard
That last point is the one that actually causes confusion in practice. The UK is on GMT in winter but on British Summer Time, UTC+1, from late March to late October. "GMT" in a meeting invite sent in July is very often meant as "London time", which is not what it says.
Which to Use
Use UTC for anything written down or machine-read: timestamps, logs, databases, APIs, scheduled jobs, flight times, and any coordination across countries. It has no daylight saving and no seasonal ambiguity, which is the entire reason it exists.
GMT is fine in conversation, in historical references, and when you mean the UK's winter time zone specifically — provided everyone reading knows whether BST is in effect.
Offsets From Zero
Both serve as the zero point the world's time zones are defined against. Every other zone is expressed as an offset:
- Eastern Standard Time is UTC-5
- Central European Time is UTC+1
- India Standard Time is UTC+5:30
- Japan Standard Time is UTC+9
Lead With UTC
When proposing a time across several countries, give UTC first and local times after it: "14:00 UTC — 9:00 AM in New York, 2:00 PM in London, 7:30 PM in Delhi." Everyone then has one unambiguous reference to check against, and nobody has to work out whether you meant GMT or BST.
The Short Version
UTC and GMT describe nearly the same instant by different means — one by atomic clocks, one by the sun over a particular hill in London. The gap between them is under a second, so for scheduling anything they're interchangeable.
What isn't interchangeable is what each implies. UTC is a fixed standard that never shifts. GMT, in common use, usually means British local time, which shifts by an hour every summer. If you want to be understood without ambiguity, write UTC — the TimeZonder converter will show you what it is anywhere else.