London Time Zone: GMT, BST, and the History of Time at the Prime Meridian

What Is the Time Zone in London Called? GMT and BST Explained

A meeting set for “3 PM London time” in July is not 3 PM GMT. It is 4 PM BST. That one-hour gap trips up more callers than any other detail.

London runs on two legal times. From the last Sunday in October to the last Sunday in March, it is Greenwich Mean Time (GMT), which sits at UTC+0. For the remaining months, British Summer Time (BST) pushes the offset to UTC+1. The switch is not subtle. In 2026, clocks jump forward on Sunday 29 March at 01:00 GMT, becoming 02:00 BST. They fall back on Sunday 25 October at 02:00 BST, reverting to 01:00 GMT. The 2027 dates are 28 March and 31 October.

So London is never a fixed offset. Treat it as one and you will be an hour out for seven months of the year.

Which one is it right now?

Late March to late October: BST. Late October to late March: GMT. The live clock on the homepage shows the correct abbreviation for this second.

GMT vs UTC: Why the Difference Matters for London

GMT is a time zone. UTC is a time standard. They share an offset but not a job.

GMT is the mean solar time measured at the Royal Observatory in Greenwich. It is the legal winter time of the United Kingdom. UTC is an atomic-time standard that never shifts for daylight saving. It is the backbone the world synchronises to.

When London observes GMT, the local time reads the same as UTC. When London observes BST, local time runs one hour ahead of UTC. Never write “London is on UTC.” Write “London is on GMT (UTC+0)” or “London is on BST (UTC+1).” International broadcasters like the BBC World Service schedule in UTC and often label it “GMT” year-round, which is why a programme listed as “09:00 GMT” in July means 10:00 BST in London.

The IANA Time Zone: Europe/London and What It Encodes

Computers do not use “GMT” or “BST” as labels. They use Europe/London.

This single IANA identifier holds every transition the UK has ever made: the annual GMT-to-BST shifts, the wartime British Double Summer Time (UTC+2), and the abandoned 1968-1971 experiment with year-round BST. Set a device to Europe/London and it will handle every past and future change correctly. Do not hard-code a fixed offset. Do not pick a generic “GMT” zone. Use Europe/London.

A Brief History: How Greenwich Became the Centre of World Time

Before the 1840s, British towns ran on local solar noon. Bristol’s noon sat roughly 10 minutes behind London’s. That worked until the railways arrived.

In 1840 the Great Western Railway adopted “London Time” (GMT) for its timetables. Other companies followed. By 1847 the Railway Clearing House pushed for a single standard across all lines. The Statutes (Definition of Time) Act 1880 then made GMT the legal time for the whole of the UK. Four years later, the International Meridian Conference chose the meridian at the Royal Observatory, Greenwich as 0° longitude. London became the literal zero point for global timekeeping and navigation.

The Prime Meridian and the Royal Observatory, Greenwich

The Royal Observatory in Greenwich sits at 0° longitude. GMT is the mean solar time measured there. When the sun crosses the meridian directly overhead, it is 12:00 noon GMT. A brass line in the courtyard marks the meridian. Stand on it and you straddle the reference line for every time zone on Earth.

BST Abbreviation Confusion: British, Bangladesh, and Bougainville Standard Time

“BST” is overloaded. It can mean British Summer Time (UTC+1), Bangladesh Standard Time (UTC+6), or Bougainville Standard Time (UTC+11). A reader in Dhaka sees “BST” and thinks UTC+6. Always write “British Summer Time” in full the first time you use it, or pair it with the offset: BST (UTC+1).

Common Misconceptions About the London Time Zone

“London is always on GMT.” False. Using GMT as a year-round label for UK time creates a one-hour error every summer. A call booked for “3 PM GMT” in July is an hour late.

“GMT and UTC are the same thing.” They share an offset. GMT is a time zone that gives way to BST in summer. UTC is a fixed standard that never shifts.

“Scotland has a different time zone.” It does not. The whole UK uses Europe/London. The political argument about dark winter mornings in Scotland is a debate about changing the rules, not a current legal difference.

“The clocks change on a fixed date.” They change on the last Sunday of March and the last Sunday of October. North America moves on the second Sunday in March and the first Sunday in November. For a few weeks each spring and autumn, the transatlantic time gap is off by an hour.

“UK transport uses the 12-hour clock.” Conversation does. Timetables and official communications use the 24-hour clock. A train at “14:30” is 2:30 PM.

Practical Recommendation

Before you schedule anything, check whether London is on GMT or BST. The live clock on the homepage shows the current time and offset right now. For future dates, use the clock-change dates above.

For detailed clock-change rules, see the daylight saving time page. For time differences with specific cities, use the converter on the homepage. For the full history, including Double Summer Time and the 1968-1971 trial, see the history page.