What does GMT mean?
Greenwich Mean Time — a time zone, not a standard, and one that stops describing British local time for seven months of every year.
The short answer
GMT is Greenwich Mean Time: UTC+0. It is the UK's winter clock and the zero point that other offsets are counted from — “GMT+5” means five hours ahead of it.
The catch: from late March to late October the UK is on BST (UTC+1), so for most of the year “GMT” does not describe the time in London.
Where the name comes from
Mean solar time at the Royal Observatory in Greenwich, London.
"Mean" is doing real work in that phrase. True solar time varies through the year because the earth's orbit is elliptical and its axis tilted — solar noon drifts by up to about 16 minutes either side of clock noon. Mean time averages that out into a uniform 24-hour day.
Greenwich became the reference at the 1884 International Meridian Conference, largely because most of the world's shipping already used charts based on it.
GMT as an offset versus GMT as a zone
Two different uses, and conflating them causes most of the confusion.
As a reference point: "GMT+5:30" simply means five and a half hours ahead of the zero meridian. Perfectly clear, and widely used — many devices display offsets this way.
As a zone: GMT is the legal time of the UK, Ireland and Portugal in winter, and of several West African countries year-round.
The first use is always valid. The second is only valid in winter.
Why "GMT" in July is wrong
The UK moves to British Summer Time, UTC+1, from the last Sunday in March to the last Sunday in October — this year 2026-03-29 to 2026-10-25.
So a meeting written as "3pm GMT" in July is the British equivalent of writing "3pm EST" in July: it names a winter clock during summer. Either the reader mentally corrects it, or a tool takes it literally and everyone is an hour out.
Safe alternatives: "UK time", or simply "London".
GMT versus UTC
Interchangeable for scheduling; different in kind.
UTC is an atomic standard with no national meaning. GMT is a time zone with a country attached and a seasonal alternative. For written timestamps, UTC is the better choice precisely because it does not drift into meaning "whatever Britain is on."
Where GMT is used year-round
Several countries sit at UTC+0 permanently and do not change their clocks: Iceland, Ghana, Senegal, Mali, Côte d'Ivoire, Burkina Faso and others across West Africa.
For those places "GMT" is correct in every month — one of the few contexts where the abbreviation is unambiguous all year.
Related: What is UTC time? · Convert EST to GMT
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Figures on this page are computed from the IANA time zone database. Last generated 2026-08-18.