GMT vs UTC: What’s the Difference?
In everyday use, GMT and UTC mean the same thing. Technically, they’re defined differently — and GMT is also a real time zone that changes with the seasons.
For almost all everyday purposes, GMT (Greenwich Mean Time) and UTC (Coordinated Universal Time) are the same: both sit at UTC+0. That’s why you’ll see them used interchangeably in casual writing, old software, and radio time signals.
The technical difference
GMT is historically an astronomical time standard, based on the position of the sun at the Royal Observatory in Greenwich, London. UTC is defined using atomic clocks, with occasional leap seconds added to stay aligned with the Earth’s actual rotation. In practice the two never drift apart by more than a fraction of a second.
GMT is also a real, living time zone
Unlike UTC, GMT is the actual civil time zone used in the United Kingdom during winter. In summer, the UK switches to British Summer Time (BST, UTC+1) for daylight saving. So "GMT" can mean two different things depending on context: the fixed UTC+0 reference, or the seasonal civil time zone of London and other countries that follow the same rule.
This is why Timemazone stores London’s time zone as the IANA identifier Europe/London rather than "GMT" — the identifier correctly captures both the winter GMT period and the summer BST period, while "GMT" alone would be ambiguous about which one you mean.