GMT — Greenwich Mean Time
Mean solar time at the Royal Observatory in Greenwich, London.
GMT stands for Greenwich Mean Time. Mean solar time at the Royal Observatory in Greenwich, London.
GMT is UTC+00:00 from Coordinated Universal Time (UTC). The primary IANA zone is Etc/GMT.
GMT is used year-round without daylight saving adjustments.
Standard / daylight pair
BST · UTC+1 — March–October in the United Kingdom
Where this abbreviation is used
| Meaning | Offset | Region |
|---|---|---|
| Greenwich Mean Time | UTC+0 | United Kingdom, Ireland, Portugal, Iceland, and West Africa |
Countries using this zone
- United Kingdom
- Ireland
- Portugal
- Iceland
- Ghana
- Senegal
- Mali
- Mauritania
- The Gambia
- Guinea
- Sierra Leone
- Liberia
- Cote d'Ivoire
- Togo
- Burkina Faso
Principal cities
- London
- Lisbon
- Accra
- Reykjavik
- Dakar
- Abidjan
History
The International Meridian Conference in October 1884 established the Royal Observatory in Greenwich as the site of the prime meridian, creating a global standard for navigation. Although UTC replaced GMT as the scientific basis for timekeeping in 1972, the term remains the legal and popular name for the time zone in many regions. Most West African nations using this zone have never adopted daylight saving time, maintaining a consistent offset for decades.
Frequently Asked Questions
GMT — Greenwich Mean Time. Mean solar time at the Royal Observatory in Greenwich, London.
GMT is UTC+00:00 year-round.
Countries and territories primarily using GMT include: GB.
GMT is used year-round without daylight saving adjustments.
The primary IANA time zone representing GMT is Etc/GMT. Operating systems and programming libraries use this identifier to resolve local time.
UTC is a high-precision atomic time standard, while GMT is a formal time zone used by specific countries. For most practical purposes, they share the same time. However, UTC is the global scientific reference, whereas GMT is a civil designation applied to countries near the prime meridian during their standard time period.
Greenwich Mean Time itself is a standard time and does not change. However, countries like the United Kingdom and Portugal move from GMT to daylight saving time in the spring. During these months, they use British Summer Time or Western European Summer Time, which are both 1 hour ahead of the prime meridian.
Several nations do not observe daylight saving time and remain on GMT throughout the entire year. These include Iceland in the North Atlantic and many West African countries such as Ghana, Senegal, Mali, Mauritania, and The Gambia. This provides these regions with a consistent time offset that never changes regardless of the season.
The United Kingdom uses Greenwich Mean Time as its official standard time during the winter. This period begins on the last Sunday of October at 02:00 and ends on the last Sunday of March. During the summer months, the UK switches to British Summer Time, returning to GMT when the clocks go back in autumn.
No, the United States does not use GMT as a local time zone, but it is frequently used as a reference for international coordination. All US time zones are calculated as negative offsets from the prime meridian. For example, Eastern Standard Time is 5 hours behind GMT, while Pacific Standard Time is 8 hours behind.