What Is GMT (Greenwich Mean Time)?
Last updated September 25, 2026
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.