UTC vs GMT: what the difference actually is
For anything you are scheduling, UTC and GMT name the same clock reading. They are not the same kind of thing, and the difference is why one of them is safe to write down and the other is not.
Is UTC the same as GMT?
The reading is the same. The two words are not. UTC is a time standard, kept by atomic clocks and coordinated internationally; GMT is a time zone, and historically an astronomical standard based on the sun over Greenwich. Nothing you schedule will ever see them differ.
The measured difference is a fraction of a second. UTC is held within 0.9 seconds of the Earth's rotation by inserting a leap second when the gap is predicted to reach that figure, and in 2022 the international body responsible decided to raise that tolerance by or before 2035. None of this reaches a calendar entry.
Why is writing GMT a problem?
Because in the place the name comes from, it is only correct for half the year. The United Kingdom is on GMT in winter and on British Summer Time, one hour ahead, from the last Sunday in March to the last Sunday in October. A meeting written as "3pm GMT" in June is almost always meant as London time, which is 3pm BST and 2pm GMT.
The same word is also used loosely for a whole family of fixed offsets, as in GMT+2, which is a number rather than a place. The GMT zone page shows what London's clock actually reads on any date the pinned release covers.
So what should you write in an invitation?
Local time with the place named, and a calendar attachment that carries the instant. Add UTC where a written record needs one canonical reading.
- Avoid a bare GMT, a bare offset, and a bare three-letter abbreviation. Each of them is a number or a label that a reader has to guess a date for.
- Prefer "14:00 in Berlin (12:00 UTC)" over "14:00 GMT+2". The first survives a clock change; the second does not.