GMT is a time zone; UTC is the global time standard set by atomic clocks. In daily use they read the same, but UTC never changes for summer time while London (GMT) does.
GMT — Greenwich Mean Time — is the time zone historically based on the mean solar time at the Royal Observatory in Greenwich, London. It’s a civil time zone: the United Kingdom uses it in winter, and several African countries use it year-round. UTC — Coordinated Universal Time — is not a time zone at all but the world’s timekeeping standard, maintained by a global network of atomic clocks.
In everyday use the two never differ by more than a fraction of a second, which is why people treat them as interchangeable. The distinction matters in precise contexts: UTC is defined by atomic physics and adjusted with occasional leap seconds to stay aligned with Earth’s rotation, while GMT is defined by the sun’s position over one meridian. Aviation, computing, and science quote UTC exclusively; “GMT” survives mostly in broadcasting and British usage.
One common confusion: London is not always on GMT. From late March to late October, the UK observes British Summer Time (UTC+1), so “the time in London” and “GMT” part ways for half the year. If a meeting is set in GMT during a London summer, it happens an hour before London wall clocks say it should.
When you schedule across borders, the safest habit is to quote UTC and let each participant convert. The time zone converter at itsdately.com does that conversion for over 300 zones, with daylight saving handled automatically.
Frequently asked questions
Related on itsdately: UTC+5, GMT+8, +05:30 — Time Zone Offset Notation Explained · Military Time Made Simple: The Complete 24-Hour Guide · The 24-Hour Watch Dial: Time for Pilots, Poles, and Submarines
Further reading: Royal Museums Greenwich on GMT