What Is GMT (Greenwich Mean Time)?
Last updated October 18, 2026
One-line definition: GMT is UTC+0, the UK's standard time in winter, named after the Royal Observatory in Greenwich, England. In summer the UK switches to BST (UTC+1). To see what time it is in London right now, check current time in London.
What GMT Means
GMT stands for Greenwich Mean Time: mean solar time at the Royal Observatory in Greenwich, London. “Mean” means it follows an averaged sun, which smooths out the day-to-day wobble in when the real sun is highest.
British railways adopted GMT in 1847, and it became Britain's legal time in 1880. At the 1884 International Meridian Conference in Washington, 22 countries voted to make the Greenwich meridian the world's zero line. France abstained and kept using the Paris meridian until 1911.
GMT is UTC+0, which means no offset from Coordinated Universal Time. The modern prime meridian used for mapping, called the IERS Reference Meridian, actually sits about 102 metres east of the old Greenwich line, but clocks aren't affected by that.
GMT vs BST: The UK Seasonal Switch
The UK uses two timezone labels across the year:
| Period | Label | UTC Offset |
|---|---|---|
| Late October to late March (winter) | GMT | UTC+0 |
| Late March to late 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
On January 1, 1972, UTC replaced GMT as the international civil time standard. GMT follows the Earth's slightly irregular rotation, while UTC is computed from atomic clocks and nudged by leap seconds to stay close to it.
In practice the two differ by less than a second, so for everyday scheduling they are treated as identical. For technical systems such as server timestamps and scientific measurement, UTC is the correct term. See UTC vs GMT for the full comparison.
Which Countries Use GMT Year-Round?
| On GMT (UTC+0) all year | Iceland, Ghana, Senegal, Mali, Mauritania, Guinea, Guinea-Bissau, The Gambia, Sierra Leone, Liberia, Togo, Burkina Faso, Ivory Coast, and Sao Tome and Principe (since 2018) |
|---|---|
| On GMT in winter only | United Kingdom, Ireland (summer: Irish Standard Time, UTC+1), mainland Portugal, the Faroe Islands, Madeira, and the Canary Islands |
Morocco is a recent special case. It ran UTC+1 for most of the year from 2018 and dropped to UTC+0 around Ramadan, then moved to UTC+0 all year on September 20, 2026. The time zone converter applies the correct offset (GMT or BST) for any date automatically.
The “GMT+1” Sign Trap
In plain English, GMT+1 means one hour ahead of Greenwich, the same as UTC+1. But some software flips the sign. In the tz database, a zone named Etc/GMT+1 is actually one hour behind UTC, and Etc/GMT-14 is fourteen hours ahead. The reversed signs follow the POSIX convention, where a positive number means west of Greenwich.
If a time zone name or setting says “GMT+something”, check which direction it means before you trust it. Using a city-based name like Europe/London avoids the problem. See the IANA time zone database.
Frequently Asked Questions
What time zone is GMT?
GMT is UTC+0, no offset from Coordinated Universal Time. It is the UK's winter time zone, from late October to late March. In summer, the UK uses BST (UTC+1).
Is GMT the same as UTC?
They show the same clock time, but GMT is mean solar time at Greenwich (and the everyday name for the UTC+0 zone) while UTC is a time standard computed from atomic clocks. UTC never changes for DST; the UK's clocks leave GMT for BST in summer.
What is the difference between GMT and BST?
GMT (UTC+0) is the UK's winter time. 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.
Which countries use GMT all year?
Iceland, Ghana, Senegal, Mali, Mauritania, Guinea, Guinea-Bissau, The Gambia, Sierra Leone, Liberia, Togo, Burkina Faso, Ivory Coast, and Sao Tome and Principe. The UK, Ireland, and mainland Portugal use it only in winter.
What does GMT+1 mean?
GMT+1 means one hour ahead of Greenwich Mean Time, the same as UTC+1. Nigeria is on it all year, and France, Germany, and Spain are on it in winter. The UK is on UTC+1 (BST) in summer. Be careful with software names like Etc/GMT+1, which use the reverse sign and mean UTC-1.
Why is the prime meridian in Greenwich?
At the 1884 International Meridian Conference, 22 countries voted to adopt the Greenwich meridian as the world's zero line. France abstained and kept using the Paris meridian until 1911.
Related Tools & Terms
Sources: Greenwich Mean Time (Wikipedia), UTC+00:00 (Wikipedia), Prime meridian (Wikipedia)