GMT, Greenwich Mean Time, is the time at the prime meridian (0° longitude), which passes through the Royal Observatory in Greenwich, London. As a clock time today, GMT means UTC+0. The clock above shows UTC+0. It always shows GMT, whatever the season.
Who uses GMT?
- The UK and Ireland, in winter only. From the last Sunday of October to the last Sunday of March. In summer the UK uses British Summer Time (BST, UTC+1) and Ireland uses Irish Standard Time (also UTC+1).
- All year: Iceland and several West African countries, including Ghana, Senegal, Côte d'Ivoire, Mali, Burkina Faso, Guinea, Sierra Leone, Liberia, The Gambia, Togo and Guinea-Bissau. None of them changes its clocks.
- Morocco returned to permanent UTC+0 on 20 September 2026, after years on UTC+1 for most of the year.
"London is on GMT" is only half true
People often write "GMT" for UK time all year round, but for about seven months of the year UK clocks are an hour ahead of GMT. A meeting set for "14:00 GMT" in July is 15:00 on London clocks. If you mean UK clock time, write "UK time" or "London time". If you really mean UTC+0, write "UTC". See BST vs GMT for the full explanation.
UK dates on and off GMT
London's clock changes, from the time zone database, which matches the UK government's published dates:
- Sunday 29 March 2026: at 01:00 clocks go forward to 02:00. GMT (UTC+0) becomes BST (UTC+1), and the hour in between is skipped.
- Sunday 25 October 2026: at 02:00 clocks go back to 01:00. BST (UTC+1) becomes GMT (UTC+0), and the hour from 01:00 happens twice.
- Sunday 28 March 2027: at 01:00 clocks go forward to 02:00. GMT (UTC+0) becomes BST (UTC+1), and the hour in between is skipped.
- Sunday 31 October 2027: at 02:00 clocks go back to 01:00. BST (UTC+1) becomes GMT (UTC+0), and the hour from 01:00 happens twice.
- Sunday 26 March 2028: at 01:00 clocks go forward to 02:00. GMT (UTC+0) becomes BST (UTC+1), and the hour in between is skipped.
- Sunday 29 October 2028: at 02:00 clocks go back to 01:00. BST (UTC+1) becomes GMT (UTC+0), and the hour from 01:00 happens twice.
GMT and UTC
For everyday purposes GMT and UTC are the same time. The difference is in how they are defined. GMT began as mean solar time at Greenwich, an astronomical measure. UTC is kept by atomic clocks and adjusted, so far by leap seconds, to stay within a second of the Earth's rotation. Scientists and computer systems use UTC. Our guide GMT vs UTC explains the history.