The short answer
GMT (Greenwich Mean Time) is a time zone — the local time at the Royal Observatory in Greenwich, London, historically based on the sun's position. UTC (Coordinated Universal Time) is a time standard — the modern scientific reference maintained by atomic clocks, from which every time zone in the world is defined as an offset.
In practice their clock faces agree (UTC and GMT never differ by more than a second), so for scheduling purposes "3pm GMT" and "3pm UTC" are the same moment.
Where people actually get burned
"GMT" doesn't mean "London time" in summer
This is the mistake that actually costs people meetings. The United Kingdom uses GMT only in winter. From late March to late October, London switches to BST (British Summer Time, UTC+1). So "let's meet at 3pm GMT" said by a Londoner in July usually means 3pm London time (BST) — which is 2pm GMT. Whenever someone says GMT, check whether they mean "the GMT zone" or "whatever time it is in the UK".
Time zones with "GMT" in the name
Several regions label their zones as "GMT+X" (GMT+8 for Singapore, GMT+4 for Dubai). These labels are fine — they're offsets from Greenwich — but the same zones are equally written UTC+8 and UTC+4. Same thing, different prefix.
Software means UTC
Servers, logs, databases, APIs, and aviation all run on UTC (pilots call it "Zulu time", written 1500Z). If you're debugging a timestamp or reading a flight plan, it's UTC — no daylight saving, no exceptions, ever. That stability is exactly why engineers use it.
Why UTC offsets are the safest language
Time zone names are ambiguous: "CST" means Central Standard Time (UTC-6), China Standard Time (UTC+8), and Cuba Standard Time (UTC-5). "IST" means India (UTC+5:30), Ireland (UTC+1), and Israel (UTC+2). An offset like UTC+5:30 is never ambiguous.
The gotcha: offsets change with daylight saving. New York is UTC-5 in winter and UTC-4 in summer. So the truly safe way to schedule is to name the city: "9am New York time" delegates the DST problem to whoever (or whatever software) converts it — cities always resolve to the right offset for the given date.
Cheat sheet
| Term | What it is | DST? | Use it for |
|---|---|---|---|
| UTC | Atomic time standard | Never | Servers, logs, aviation, unambiguous scheduling |
| GMT | Time zone at Greenwich (UTC+0) | Never (but the UK leaves it in summer) | Legacy usage; UK winter time |
| BST | British Summer Time (UTC+1) | Is the DST | UK, late March–late October |
| Zulu (Z) | UTC in military/aviation notation | Never | Flight plans, weather, ops |
| City names | "9am New York time" | Handled for you | Everyday scheduling — safest of all |
The practical takeaway
Schedule by city, not by abbreviation. A tool that tracks cities — like GlobalTimeZones in your Mac menu bar — resolves DST and offsets from the IANA timezone database automatically, so "what's 9am New York for Mumbai?" is a glance (6:30pm — and the availability dot tells you whether that's acceptable) instead of a Wikipedia dive. For picking the meeting time itself, see how to schedule meetings across time zones.