Fundamentals
What Is GMT? Greenwich Mean Time Explained
Where Greenwich Mean Time comes from, how it relates to UTC, and when you should use one over the other.
· 4 min read
Greenwich Mean Time (GMT) is the mean solar time at the Royal Observatory in Greenwich, London. For centuries it was the world’s reference clock, set by the moment the sun crosses the Greenwich meridian.
GMT today
GMT is still an official timezone — the United Kingdom uses it in winter, switching to British Summer Time (BST, GMT+1) in summer. So "London time" is not always GMT, which trips up a lot of people scheduling calls with the UK.
GMT vs UTC
For scheduling, GMT and UTC are functionally identical — both sit at offset zero. UTC is the modern, precise standard kept by atomic clocks; GMT is the historical, astronomy-based one. If you are coordinating software or international meetings, prefer UTC.
A common mistake
Saying "9am GMT" in July when you mean London time is wrong — London is on BST then, an hour ahead. Always check whether the location is currently observing daylight saving before quoting a GMT offset.