The UK runs on two different clocks, and the wrong one will cost you an hour. Call a London office at "14:00 GMT" in July and you will be late. The city is on British Summer Time then, one hour ahead. Get the distinction right and you never miss a connection; get it wrong and you join the long line of callers who meant UK local time but said GMT.

Here is the split, the switch dates, the history that created it, and exactly how to schedule so the hour difference does not catch you.

GMT and BST: Two Sides of UK Time

Greenwich Mean Time is the United Kingdom's standard baseline, measured from the Prime Meridian at the Royal Observatory in Greenwich, London, at 0° longitude. When the sun sits directly over that meridian, it is noon GMT.

British Summer Time is daylight saving time: GMT plus one hour, UTC+1. The UK does not move to a different zone in summer. It shifts the same zone forward to push daylight into the evening.

The split:

  • Winter (late October to late March): GMT, UTC+0
  • Summer (late March to late October): BST, UTC+1

GMT is fixed. BST is seasonal. UK local time is whichever one the calendar demands.

What Is British Summer Time?

BST is the UK's summer clock. Clocks jump forward one hour in spring and fall back in autumn, so sunrise and sunset both land an hour later. Lighter evenings, darker mornings.

The Summer Time Act 1972 defines BST as "one hour in advance of Greenwich mean time." That act, retained as UK law after Brexit, still governs the switch today.

BST is not a separate geographic zone. It is a seasonal adjustment applied to GMT. Think of it as GMT shifted for longer evenings.

When does BST start and end?

The dates follow a rule, not a fixed calendar number. They shift every year.

  • Clocks go forward: last Sunday in March at 01:00 GMT, becoming 02:00 BST
  • Clocks go back: last Sunday in October at 02:00 BST, becoming 01:00 GMT

In 2026:

  • Forward: 29 March 2026
  • Back: 25 October 2026

In 2027:

  • Forward: 28 March 2027
  • Back: 31 October 2027

The March change deletes an hour of sleep. The October change adds one back. Both happen in the small hours, when disruption to trains, the grid and phone networks is smallest. Your phone adjusts automatically; your diary needs a manual check.

What to book and what to skip during the switch week

  • Book rail tickets for the Sunday after the change only after the operator publishes the adjusted timetable. Network Rail runs clock-change checks, but some booking systems lag.
  • Skip scheduling a 09:00 Monday meeting across the Atlantic on the Monday after either switch. Give teams 24 hours to settle.
  • Arrive at Heathrow or Gatwick for a Sunday morning flight using the new time. Check your boarding pass against a live world clock, not your wall clock.
  • Skip relying on a car sat-nav clock if the system hasn't synced. Confirm against your phone.

The #1 Mistake: Calling UK Summer Time 'GMT'

Someone in New York emails a London colleague: "Let's meet at 14:00 GMT." In January, that is correct. London is on GMT. In July, London is on BST, UTC+1. "14:00 GMT" in July means 15:00 in London. The New Yorker has scheduled the meeting an hour late.

The fix: say "UK time" and let the other person confirm, or name the offset. "14:00 London time (BST)" is unambiguous. "14:00 GMT" in summer is a trap.

Some people assume GMT itself observes summer time. It does not. GMT stays at UTC+0, always. The UK switches; GMT does not.

Why Does the UK Change Its Clocks?

The modern argument is daylight utilisation. Shift clocks forward in summer, the sun sets later, people get more usable evening light. In winter, return to GMT so mornings are not dangerously dark.

The original pitch, though, targeted something narrower: sleep.

Who Proposed Daylight Saving and Why Did It Finally Pass?

William Willett, a London builder, noticed people sleeping through summer daylight. In 1907 he published The Waste of Daylight, proposing clocks move forward by 80 minutes across four spring stages and back the same way in autumn. He argued it would cut energy use, improve health and give people more evening leisure.

A private member's bill reached Parliament in 1909. A select committee examined it. Farmers and parts of the scientific community opposed it. The bill failed. Willett died in 1915 without seeing his idea become law.

World War I and the Summer Time Act 1916

War flipped the calculation. Germany introduced daylight saving in April 1916 to save fuel. The UK followed weeks later, passing the Summer Time Act 1916. BST ran from May to October that year.

Willett's staged 80-minute version never passed, but the principle did. The UK has kept some form of summer time ever since.

World War II: Double Summer Time (GMT+2)

From 1941 to 1945, the UK ran British Double Summer Time: clocks two hours ahead of GMT in summer (GMT+2) and one hour ahead in winter (GMT+1). The country was effectively on year-round BST-plus-one. The aim was maximum wartime production and lower energy consumption. After the war, a brief 1947 extension tested the idea again before the standard GMT/BST cycle returned.

The 1968-71 Experiment: Year-Round BST

The most serious challenge to the GMT/BST cycle ran from 1968 to 1971. The UK stayed on British Standard Time (BST, UTC+1) all year. No autumn fall-back, no spring jump-forward.

The experiment tested whether permanent summer time cut road accidents and improved daily life. Results were mixed. Supporters cited fewer evening collisions; critics pointed to darker winter mornings, especially for children travelling to school. The experiment ended in 1971. The GMT/BST cycle resumed.

Will the UK Abolish Clock Changes?

The European Union voted in 2019 to end seasonal clock changes, asking member states to choose permanent summer or winter time. Implementation stalled. The UK's departure from the EU removed it from that process.

The UK government has indicated no plans to change the current arrangement. A 2019 petition to scrap clock changes gathered signatures, but the official response was firm: no change.

The political problem is choosing which permanent time to adopt. Permanent GMT means darker winter evenings. Permanent BST means the sun does not rise until after 09:00 in parts of Scotland in winter. Neither option commands a majority.

How to Schedule Meetings Correctly with the UK

When the UK is on GMT (late October to late March):

  • London is UTC+0
  • New York is 5 hours behind London
  • Sydney is 11 hours ahead of London

When the UK is on BST (late March to late October):

  • London is UTC+1
  • New York is 5 hours behind London
  • Sydney is 9 hours ahead of London

The golden rule: never say "GMT" for UK time between late March and late October. Say "BST" or "UK time" and confirm the UTC offset.

For recurring meetings, pin the transition in your calendar invite. The March switch happens at 01:00 GMT, which becomes 02:00 BST, so a 09:00 meeting becomes 08:00 GMT overnight. The October switch happens at 02:00 BST, which becomes 01:00 GMT, so a 09:00 meeting becomes 10:00 GMT.

Which entrance to use for the Royal Observatory, Greenwich

  • Enter via the National Maritime Museum gates on Romney Road. The Greenwich Park gates are steeper and add 10 minutes of uphill walking.
  • Skip the midday slot on weekends. Arrive before 10:00 or after 15:00.
  • Book the Meridian Line courtyard ticket online. The walk-up queue often sells out by 11:00.

GMT vs UTC: What Is the Difference?

GMT and UTC are used interchangeably, but they are not the same thing.

GMT is a time zone, the time at the Prime Meridian. UTC (Coordinated Universal Time) is a time standard, maintained by atomic clocks at the International Bureau of Weights and Measures in Paris. UTC does not observe summer time; it is fixed.

For everyday purposes, GMT and UTC read identically: UTC+0. The difference is technical. GMT can refer to UT1, which is based on Earth's rotation, while UTC is atomic. Unless you are doing precision astronomy, they are the same.

A timestamp ending in "Z", such as 2025-01-20T01:56:25Z, is Zulu time, the military and aviation term for UTC. It is also GMT. Same offset, three names.

Which Countries Stay on GMT Year-Round?

Several nations use GMT as their permanent standard time, with no summer shift. From the IANA time zone database, the zones using the GMT metazone include Africa/Abidjan, Africa/Bissau, Africa/Monrovia, Africa/Sao_Tome, America/Danmarkshavn, Antarctica/Troll and Etc/GMT.

Key cities on GMT year-round:

  • Abidjan, Ivory Coast, population 6,321,017
  • Monrovia, Liberia, population 1,542,549
  • Abobo, Ivory Coast, population 1,340,083
  • Bouaké, Ivory Coast, population 832,371
  • Korhogo, Ivory Coast, population 440,926
  • Bissau, Guinea-Bissau, population 439,704
  • Daloa, Ivory Coast, population 421,871
  • Koumassi, Ivory Coast, population 412,282
  • San-Pédro, Ivory Coast, population 390,654
  • Gagnoa, Ivory Coast, population 277,044
  • Yamoussoukro, Ivory Coast, population 275,686
  • Sinfra, Ivory Coast, population 245,226
  • Man, Ivory Coast, population 241,969
  • Marcory, Ivory Coast, population 214,061
  • Bondoukou, Ivory Coast, population 141,568

These locations never shift their clocks. Scheduling with Abidjan or Monrovia carries no summer-time risk.

What daylight looks like in a GMT year-round city

Abidjan sits near the equator. Daylight varies little across the year.

  • March equinox (20 March 2026): sunrise 06:20, sunset 18:27, 12 hours 6 minutes of daylight
  • June solstice (21 June 2026): sunrise 06:04, sunset 18:30, 12 hours 26 minutes of daylight
  • September equinox (22 September 2026): sunrise 06:05, sunset 18:12, 12 hours 7 minutes of daylight
  • December solstice (21 December 2026): sunrise 06:19, sunset 18:08, 11 hours 49 minutes of daylight

The swing from the shortest to the longest day is under 40 minutes. No seasonal clock change, and no dramatic shift in light.

GMT vs BST: The Practical Summary

GMT is the UK's winter time. BST is the UK's summer time. The UK switches twice a year, on the last Sunday of March and the last Sunday of October.

If you are scheduling with the UK, remember: in winter, "GMT" and "UK time" are the same. In summer, they are an hour apart. Say "BST" or "UK time" and name the offset. Your meeting will start on time.

And if someone tells you the UK is on GMT in July, they are wrong. Correct them. You will save them from the one-hour error that has been tripping up international schedules for more than a century.