Greenwich Mean Time (GMT)

UTC offset: +00:00
Primary IANA identifier: Europe/London (with BST in summer)
Abbreviations: GMT, Z (Zulu), WET (in some European contexts)
Observed in: UK, Ireland, parts of West Africa, Iceland year-round, several other regions
Approximate population: roughly 350 million

Greenwich Mean Time started as a very local thing. The mean solar time at the Royal Observatory in Greenwich, London. Somehow it ended up as the reference point against which every clock on Earth is set. That's an outsized legacy for a small hill on the south bank of the Thames.

The offset is zero. Nothing added, nothing subtracted. When it's 3 p.m. GMT, it's 3 p.m. at the Prime Meridian, which conveniently runs through the observatory's transit telescope. Everything else on the planet is measured relative to that line.

GMT and UTC get used interchangeably in casual conversation, but they aren't quite the same thing. GMT is based on astronomical observation. UTC is based on atomic clocks, with occasional leap seconds added to keep the two roughly in sync. For day-to-day purposes the difference is negligible. For navigation, scientific work, and broadcasting, the distinction matters.

Why Greenwich

The Royal Observatory was founded in 1675 by Charles II, primarily to solve the longitude problem. That meant figuring out where ships were in the open ocean. Latitude was easy enough. You measured the sun's height at noon. Longitude required knowing the precise time at a reference point and comparing it to local time, which meant you needed reliable clocks and a reliable reference.

By the mid-19th century, Britain's maritime dominance meant ship captains around the world were already using charts based on Greenwich. The 1884 International Meridian Conference in Washington, D.C. formalized what was already common practice. Delegates from 25 countries voted to make the Greenwich meridian the world's Prime Meridian. France abstained. France actually held out using its own Paris meridian until 1911.

The British railways had adopted "Railway Time," essentially GMT, back in 1847. That replaced the patchwork of local times that had made train schedules a guessing game. By 1880, the Statutes (Definition of Time) Act made GMT the legal time across Great Britain.

One subtle change happened in 1925. Until then, the astronomical convention put noon at the start of the day. 0 hours GMT was actually local noon. That meant astronomical days ran from noon to noon, which made sense for tracking stars overhead but didn't match civil practice. The shift to a midnight-based day in 1925 finally aligned GMT with how ordinary people thought about time.

Where GMT Applies

GMT itself is a year-round, no-DST offset. The places that observe it permanently are mostly clustered in West Africa and a few isolated spots:

• Iceland, on UTC+00:00 year-round despite sitting far to the west longitudinally. Iceland abolished DST in 1968 and just settled on Greenwich time permanently. This gives Reykjavík winters with very late sunrise (around 11 a.m. in December) but the trade-off was deemed worth it for the stable, predictable clock.
• Burkina Faso, Côte d'Ivoire, The Gambia, Ghana, Guinea, Guinea-Bissau, Liberia, Mali, Mauritania, Saint Helena, São Tomé and Príncipe, Senegal, Sierra Leone, Togo, most of West Africa runs on GMT year-round without DST.
• Faroe Islands, uses GMT in winter and shifts to UTC+01:00 in summer.

The UK and Ireland are the two big GMT users that don't actually stay on GMT all year. They both shift forward an hour for daylight saving. British Summer Time (BST) and Irish Standard Time (IST). For roughly seven months of the year, London is not on GMT.

DST and the UK Pattern

In Britain, the clocks go forward on the last Sunday in March and back on the last Sunday in October. The summer offset of UTC+01:00 is called British Summer Time. The change has been in place since 1972 in its current form, though Britain experimented with various DST schemes during both World Wars.

There's a recurring debate about whether the UK should adopt permanent BST. Sometimes the idea is paired with an additional hour ahead in summer, making it UTC+02:00, called "Single Double Summer Time." The arguments for and against tend to break down along regional lines. England largely favors more evening light. Scotland, further north and west, ends up with very late sunrises if DST is permanent. December sunrise in northern Scotland under permanent BST would be close to 10 a.m., which sailors and farmers find untenable.

Ireland passed legislation supporting the EU's proposed end to seasonal time changes, but the EU plan has been stalled for years. So clocks in Dublin keep shifting in March and October.

Major Cities

London is the obvious anchor. About 9 million in the city proper and around 14 million in the broader metro area. It's the financial center of Europe (still, despite Brexit), the British government's seat, and the cultural capital of the Anglophone world. When London markets open at 8 a.m. local time, they're starting the working day for half the planet.

Dublin is the second most prominent GMT city, with around 1.4 million in the greater metro area. Ireland's economy has been transformed over the past three decades by tech and pharma investment, much of it from US firms taking advantage of EU access and English-speaking talent.

Edinburgh and Glasgow anchor Scotland. Both run on the same clock as London but experience noticeably different daylight patterns due to latitude.

Reykjavík, despite Iceland's small population (around 400,000 nationally), is a meaningful financial and tourism hub at GMT.

Accra, Ghana, is the largest West African city on GMT. About 2.5 million in the city and over 4 million in the broader area. Ghana's economy is one of the most stable in West Africa, and Accra functions as a regional hub for finance, shipping, and media.

Dakar, Senegal, plays a similar role for francophone West Africa, with about 1.1 million residents.

Abidjan, Côte d'Ivoire, is the largest of the GMT-aligned cities in West Africa at roughly 5 million metro population. It's the financial center of the francophone West African economic community (UEMOA).

How GMT Shapes Daily Life and Business

For the UK, the relationship with GMT is paradoxical. Britain invented the system, gave the world its reference time, and then routinely doesn't actually use it. From late March through late October, London is on BST. Only during the winter is the country really on GMT.

For West Africa, the practical effect is a stable, predictable clock that doesn't shift twice a year. Schools, businesses, and broadcasters never have to adjust. The latitude (most of these countries are between 5 and 15 degrees north) means that day length barely changes across seasons, so there's no daylight pattern that DST could meaningfully improve.

For the global business community, GMT remains the implicit baseline. Server logs are typically in UTC. Aviation operates in Zulu time, which is functionally identical to UTC with a one-letter NATO phonetic designator. Maritime navigation, scientific publications, and military coordination all default to GMT or UTC.

The London financial day, 8 a.m. to 5 p.m. GMT in winter and 8 a.m. to 5 p.m. BST in summer, sits in a sweet spot of overlap. It catches the tail end of the Tokyo session in the morning and the opening of New York in the afternoon. This is no accident. It's why London became and remained a global financial center.

Comparing GMT to Adjacent Zones

Western European Time (WET) is essentially GMT under a different name. It's used by Portugal, parts of Spain (Canary Islands), and the Faroe Islands during winter. WET shifts to Western European Summer Time (WEST) at UTC+01:00 in summer, matching what BST does for the UK.

Central European Time (CET) at UTC+01:00 sits one hour ahead of GMT. Paris is one hour ahead of London. This was actually a wartime imposition for France. During German occupation in WWII, France was moved to Berlin time, and the country never moved back.

Eastern European Time (EET) at UTC+02:00 is two hours ahead of GMT. Athens, Helsinki, and Cairo all sit there.

Coordinated Universal Time (UTC) is the modern technical successor to GMT. For all civilian purposes the two are interchangeable. Astronomers and physicists care about the distinction, because UTC includes leap seconds and GMT doesn't. For ordinary life, the difference is fractions of a second.

Quirks Worth Knowing

The Prime Meridian as marked on the ground at Greenwich is actually about 100 meters off from the modern reference meridian used by GPS satellites. The original was based on the transit telescope's optical axis. The modern one accounts for refinements in how Earth's shape is measured. Tourists who line up to stand on the brass strip at Greenwich are technically standing on the old line, not the satellite-defined one.

Antarctica's research stations don't really observe any time zone in the conventional sense. Most stations use the time zone of the country that operates them. So an American station might run on McMurdo's New Zealand time, while a UK station might run on GMT. It's pragmatic rather than geographical.

The "GMT" notation gets misused frequently to mean "Coordinated Universal Time" in computing contexts. Most server software has either deprecated or aliased "GMT" to UTC. Email headers still routinely show "GMT" in date stamps.

Technical Identifiers

In IANA Time Zone Database terms, the relevant entries are:

• Europe/London, UK time, GMT in winter and BST in summer
• Europe/Dublin, Irish time, GMT and IST
• Africa/Accra, Africa/Abidjan, Africa/Dakar, and other West African GMT zones
• Atlantic/Reykjavik, Iceland on permanent GMT
• Etc/GMT, fixed UTC+00:00 with no DST, for technical use

Quick Reference

Attribute	Value
Standard offset	UTC+00:00
DST in UK and Ireland	UTC+01:00 (BST or IST)
DST start (UK)	Last Sunday in March
DST end (UK)	Last Sunday in October
Primary IANA zone	Europe/London
Year-round GMT countries	~15 (most in West Africa, plus Iceland)
Largest GMT city	London (~9M)
Relationship to UTC	Functionally identical for civilian use