GMT vs UTC: What’s the Difference?

GMT (Greenwich Mean Time) and UTC (Coordinated Universal Time) usually show the same time, so the terms get used interchangeably. Technically they are different: GMT is a time zone that some countries actually observe, while UTC is the international time standard that all time zones are measured from.

Greenwich Mean Time

GMT was established in the 19th century, based on the mean solar time at the Royal Observatory in Greenwich, London. It became the world’s reference for time and longitude during the era of railways and global shipping.

Today GMT is still a legal time zone — the United Kingdom uses it in winter (switching to British Summer Time, UTC+1, in summer), and several countries in West Africa observe it year round.

Coordinated Universal Time

UTC replaced GMT as the world’s technical time reference in 1972. Rather than being based on the Sun’s position, UTC is derived from atomic clocks, with occasional leap seconds to stay in step with the Earth’s rotation.

Crucially, UTC is not a time zone anyone lives in — it is a baseline. Saying a server runs "in UTC" means it uses the standard directly, without any regional daylight-saving rules.

Which should you use?

For scheduling, software, and anything international, prefer UTC — it is unambiguous and standard. Use GMT when you specifically mean the UK/West Africa time zone, or in casual British usage where the distinction rarely matters.