UTC and GMT are not the same thing
One is a time standard that never shifts. The other is a time zone that springs forward every year. Treating them as synonyms is correct in January and wrong in July.
The short version
UTC never changes. It is the reference every other time is expressed against — an offset of exactly zero, all year, forever. Nothing observes daylight saving on UTC, because UTC is not a place.
GMT, as people normally use it, does change. In everyday speech “GMT” usually means the time in the UK — and the UK moves its clocks. So for roughly half the year the thing people call GMT is one hour ahead of UTC, and a meeting booked “10am GMT” in July is very likely an hour away from what the sender pictured.
The demonstration
This is not a subtlety you have to take on trust. Ask any browser what offset the
Europe/London zone is running at, and give it two different instants:
- At 15 January 2026, 12:00 UTC —
Europe/Londonreports GMT. Offset zero. Identical to UTC. - At 15 July 2026, 12:00 UTC — the same zone, unchanged, reports GMT+01:00.
Same zone. Same query. Two different answers, because the question “what is the offset?” is incomplete without “when?”. That is the entire content of the confusion.
Why the two get muddled
Historically they were closer to interchangeable. Greenwich Mean Time was a mean solar time measured at Greenwich, and it did the job UTC does now: the thing everything else was counted from. UTC replaced it as the reference because it is defined by atomic clocks rather than by the Earth’s rotation, which turns out to be slightly irregular.
What survived was the word. “GMT” stayed in circulation as a label for UK civil time, in email footers, on conference invitations, in the names of file formats. It is the same word doing a different job, and the difference only shows up in summer.
What to write instead
If you mean the standard that never moves, write UTC. If you mean the time
someone in a particular place will read off their own clock, name the place —
Europe/London, America/New_York, Asia/Singapore — and let
their device work out the offset for the date in question.
The named-place form is the one that survives contact with reality. It stays correct across a daylight-saving transition, across a government changing its mind about daylight saving, and across the reader being somewhere you did not anticipate. An offset written into a message is a snapshot; a zone name is an instruction.
Where this bites in practice
The failure is quiet, which is what makes it expensive. A recurring meeting set up in winter as “4pm GMT” does not announce itself as broken in spring; it simply starts happening an hour away from where half the attendees expect it, and everyone assumes someone else made the mistake. Log timestamps labelled GMT drift the same way, and the drift is invisible until two systems are compared.
Our time zone converter works in named zones for this reason, and the meeting planner compares zones as instants rather than as fixed offsets. There is a fuller explanation of that distinction in a time zone is not an offset.