GMT to UTC
London to the prime meridian.
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All date & timeQuestions
What time is UTC when it is 12:00 GMT?
12:00. Coordinated Universal Time is no difference the same time as Greenwich Mean Time in January and 1 hour behind it in July. The gap moves because London observes daylight saving and the prime meridian does not, so the difference between them is not a fixed number. A 9-to-5 day in London overlaps a 9-to-5 day in the prime meridian for 8 hours, between 09:00 and 17:00 GMT.
Is the difference between GMT and UTC the same all year?
No, and this is the pair where that matters. Coordinated Universal Time is no difference the same time as Greenwich Mean Time in January and 1 hour behind it in July. The gap moves because London observes daylight saving and the prime meridian does not, so the difference between them is not a fixed number. Booking a recurring call across these two and assuming one number will drift by an hour twice a year — check the date, not just the zones.
What do GMT and UTC actually stand for?
GMT is Greenwich Mean Time, the zone covering London, currently UTC+0 in January. UTC is Coordinated Universal Time, covering the prime meridian, at UTC+0. GMT and UK time are the same thing only in winter. From late March to late October London runs on British Summer Time, an hour ahead of GMT, so a meeting booked as 'GMT' with someone in London is an hour out for most of the year.
Anything worth knowing about Coordinated Universal Time?
UTC never changes for daylight saving, which is why servers, aviation and broadcasting all run on it. It is the reference every other zone is defined against, including GMT — the two are the same clock time, but GMT is a time zone and UTC is a time standard.
When is a good time to call between London and the prime meridian?
A 9-to-5 day in London overlaps a 9-to-5 day in the prime meridian for 8 hours, between 09:00 and 17:00 GMT. If the call has to happen outside that, the fairer arrangement is usually to alternate which side takes the awkward hour rather than always asking the same one.
How it works, and more about gmt → utc
The difference, and whether you can rely on it
Greenwich Mean Time and Coordinated Universal Time read the same clock in January. In July it is 1 hour behind instead. That is the part worth carrying away: this is not a pair you can memorise a single number for.
Why the gap moves
London observes daylight saving; the prime meridian does not. So the clocks in one city move twice a year while the other stays put, and the distance between them changes with them. Any calculation you did in one season is wrong in the other. The converter above uses the date you give it rather than today’s offset, which is the only way to get a future meeting right when it falls on the other side of a changeover.
When you can both be at a desk
A standard working day in London overlaps one in the prime meridian between 09:00 and 17:00 GMT — 8 hours. Anything outside that asks somebody to start early or finish late.
Reading the abbreviations literally will mislead you
GMT strictly means one fixed offset, but almost nobody uses it that way. People write it to mean the time in London, which shifts if that city observes daylight saving — and it does. This page follows the common usage rather than the strict one, because the strict reading gives an answer that is wrong for a good part of the year.
GMT and UK time are the same thing only in winter. From late March to late October London runs on British Summer Time, an hour ahead of GMT, so a meeting booked as 'GMT' with someone in London is an hour out for most of the year. UTC never changes for daylight saving, which is why servers, aviation and broadcasting all run on it. It is the reference every other zone is defined against, including GMT — the two are the same clock time, but GMT is a time zone and UTC is a time standard.
Scheduling something recurring
For a one-off, the converter above is the whole job. For a standing meeting across these two zones, where the gap changes, it is worth putting the invitation in a calendar that stores the zone rather than the time — then the changeover is handled for you and nobody has to remember. If you are trying to find a slot across more than two places at once, the meeting planner shows a whole day at a glance instead.