What Is GMT (Greenwich Mean Time)?
One-line definition: GMT is UTC+0 — the UK's standard timezone in winter, named after the Royal Observatory in Greenwich, England. In summer, the UK switches to BST (UTC+1).
What GMT Means
GMT stands for Greenwich Mean Time. The name comes from the Royal Observatory in Greenwich, London — the location chosen as the prime meridian (0° longitude) at the 1884 International Meridian Conference. “Mean time” refers to mean solar time at that meridian — the average time of solar noon over the year.
GMT is UTC+0. That means no offset from Coordinated Universal Time. When London is on GMT (October to March), local noon aligns closely with solar noon at the Greenwich Observatory.
GMT vs BST — The UK Seasonal Switch
The UK uses two timezone labels across the year:
| Period | Label | UTC Offset |
|---|---|---|
| October → late March (winter) | GMT | UTC+0 |
| Late March → October (summer) | BST (British Summer Time) | UTC+1 |
This is important: if someone says “the meeting is at 3 PM GMT” in June, they might mean 3 PM UK local time — which is actually BST (UTC+1), not GMT (UTC+0). That's a one-hour error. To avoid ambiguity, use UTC or specify whether you mean GMT or BST.
GMT vs UTC — The Technical Difference
GMT was replaced by UTC as the international time standard in 1972. The reason: GMT is based on astronomical observation of the sun, which is slightly irregular. UTC is based on atomic clocks — far more stable.
In practice, GMT and UTC differ by less than a second. For everyday scheduling, they are treated as identical. For technical systems — satellite navigation, scientific measurement, server timestamps — UTC is the correct term. See UTC vs GMT for the full comparison.
Which Countries Use GMT Year-Round?
Several countries stay on UTC+0 all year without observing Daylight Saving Time:
- 🇮🇸 Iceland — UTC+0 year-round
- 🇬🇭 Ghana — UTC+0 year-round
- 🇸🇳 Senegal — UTC+0 year-round
- 🇲🇱 Mali — UTC+0 year-round
- 🇲🇷 Mauritania — UTC+0 year-round
- 🇬🇳 Guinea — UTC+0 year-round
- 🇬🇧 UK — GMT in winter, BST (UTC+1) in summer
- 🇮🇪 Ireland — GMT in winter, IST (UTC+1) in summer
The [time zone converter](/time-zone-converter) uses the IANA timezone database and automatically applies the correct offset (GMT or BST) based on today's date.
Frequently Asked Questions
What time zone is GMT?
GMT is UTC+0 — no offset from Coordinated Universal Time. It is the UK's winter timezone (October to March). In summer, the UK uses BST (UTC+1).
Is GMT the same as UTC?
They show the same clock time, but GMT is a timezone based on solar time and UTC is an atomic time standard. UTC never changes for DST; the UK's "GMT" label switches to BST in summer.
What is the difference between GMT and BST?
GMT (UTC+0) is the UK's winter timezone. BST (British Summer Time, UTC+1) is one hour ahead. The UK switches from GMT to BST on the last Sunday in March and back on the last Sunday in October.
What does GMT+1 mean?
GMT+1 means one hour ahead of Greenwich Mean Time — the same as UTC+1. Countries on GMT+1 include France, Germany, Spain, and Nigeria. The UK is on GMT+1 (BST) during summer.