This guide covers Greenwich Mean Time: History | TimeFlow — what it means, how it works, and how to use it. The explanations below are followed by worked examples with real dates and real times, a set of frequently asked questions, and links to the tools that do the calculation for you: the time zone converter, the world clock, and the UTC converter. Where a number is given — an offset, a date, a count — it is computed from the IANA Time Zone Database for the date stated, not copied from a table that may be out of date.

Try the tool: Time Zone Converter — convert any time between any two cities, with the daylight saving rules for the date you choose.

Greenwich Mean Time began as a practical necessity for astronomers and sailors. The Royal Observatory at Greenwich was founded in 1675 to solve the problem of longitude at sea, and its astronomers measured time by watching the stars cross the meridian. By the 19th century, Greenwich time was the reference for British navigation, and the Royal Navy's chronometers were set to it.

The railway made it national. In 1847 the Railway Clearing House adopted Greenwich time for all British railways, and in 1880 the Statutes (Definition of Time) Act made GMT the legal time of Great Britain — the first time any country had a statutory standard time. The rest of the world followed: the 1884 International Meridian Conference chose Greenwich as the Prime Meridian, and GMT became the world's time standard.

GMT's reign lasted until 1972, when it was formally replaced by UTC (Coordinated Universal Time), which is based on atomic clocks rather than the Earth's rotation. The change was technical, not practical: UTC and GMT show the same time on the clock, and UTC is kept in step with the Earth's rotation by leap seconds.

GMT never went away in everyday language. Britain still calls its winter time GMT, the BBC still announces it, and the term is used worldwide to mean UTC+0. The Royal Observatory still stands on the Prime Meridian, and visitors can stand with one foot in the eastern hemisphere and one in the western — on the line that divides the world's time.

In practice

Greenwich became the world's time reference because Britain was the world's greatest maritime power. The Royal Observatory at Greenwich, founded in 1675, published the nautical tables that ships used to find their longitude — and longitude is time: the difference between local time and Greenwich time tells you how far east or west you are. By the 19th century, most of the world's shipping already used Greenwich as its reference.

The 1884 conference formalised what the sailors already did. Twenty-five nations voted for Greenwich as the prime meridian, over Paris, and the world divided into time zones measured from it. The choice was practical, not scientific: Greenwich was where the tables were, and where the ships were. The meridian itself is marked by a brass strip in the Observatory courtyard — though the modern meridian, defined by GPS, is about 100 metres to the east.

GMT's role as the world standard ended in 1972, when UTC replaced it, but the name survives. Britain's winter time is still legally called GMT, the BBC still announces it, and the term is used worldwide to mean UTC+0. The observatory, the meridian and the name are all still there — only the clocks have changed, from solar observations to atomic time.

The numbers: daylight saving in 2026

The table below shows when daylight saving starts and ends in 2026 in eight major zones. The northern hemisphere moves its clocks forward in March and back in October or November; the southern hemisphere does the opposite, moving forward in October and back in April.

Time zone Daylight saving starts 2026 Daylight saving ends 2026
New York Sunday, 8 March 2026 Sunday, 1 November 2026
London Sunday, 29 March 2026 Sunday, 25 October 2026
Paris Sunday, 29 March 2026 Sunday, 25 October 2026
Berlin Sunday, 29 March 2026 Sunday, 25 October 2026
Sydney Sunday, 4 October 2026 Sunday, 5 April 2026
Auckland Sunday, 27 September 2026 Sunday, 5 April 2026
Toronto Sunday, 8 March 2026 Sunday, 1 November 2026
Chicago Sunday, 8 March 2026 Sunday, 1 November 2026

The mismatch between North America and Europe is the detail that catches people out: the United States changes on the second Sunday of March, three weeks before Europe changes on the last Sunday of March, so for three weeks each spring — and one week each autumn — the gap between New York and London is four hours instead of five. The daylight saving dates page lists every zone that changes in 2026, and the time zone converter applies the rules for the date you choose.

The same eight zones in 2027:

Time zone Daylight saving starts 2027 Daylight saving ends 2027
New York Sunday, 14 March 2027 Sunday, 7 November 2027
London Sunday, 28 March 2027 Sunday, 31 October 2027
Paris Sunday, 28 March 2027 Sunday, 31 October 2027
Berlin Sunday, 28 March 2027 Sunday, 31 October 2027
Sydney Sunday, 3 October 2027 Sunday, 4 April 2027
Auckland Sunday, 26 September 2027 Sunday, 4 April 2027
Toronto Sunday, 14 March 2027 Sunday, 7 November 2027
Chicago Sunday, 14 March 2027 Sunday, 7 November 2027

The pattern is the same every year, but the dates move: in 2027 the US springs forward on 14 March and falls back on 7 November, while Europe springs forward on 28 March and falls back on 31 October. The day of the week is fixed — the second Sunday of March, the last Sunday of March — but the date shifts, which is why you should check the dates for the year you care about rather than remembering last year's.

Key facts

  • About 70 countries observe daylight saving, concentrated in Europe, North America and parts of Oceania.
  • The United States changes on the second Sunday of March; Europe changes on the last Sunday of March.
  • Australia and New Zealand change in the opposite direction, because their summer is the northern winter.
  • Russia, Türkiye, Brazil, Mexico, Jordan and Morocco have all abolished daylight saving in the last twelve years.
  • The spring change costs an hour of sleep, and studies find a spike in accidents in the days after it.

Frequently asked questions

When was GMT created?

The Royal Observatory at Greenwich was founded in 1675, and GMT became the world's time standard at the International Meridian Conference in 1884.

Why Greenwich?

Because Britain was the world's greatest maritime power, most ships already used Greenwich for navigation, and the Observatory had published nautical tables for a century.

Where is the Prime Meridian?

At the Royal Observatory in Greenwich, London — marked by a brass strip in the courtyard. The modern meridian defined by GPS is about 100 metres east of it.

When did UTC replace GMT?

In 1972, when UTC formally became the world standard. GMT had been the standard since 1884, but atomic clocks are more precise than solar observations.

Is GMT still used today?

Yes, as a name: Britain's winter time is legally GMT, the BBC announces it, and the term is used worldwide to mean UTC+0.

When does daylight saving start in 2027?

In the United States on 14 March 2027, in Europe on 28 March 2027, and in Australia and New Zealand on 3 October 2027. The daylight saving dates page lists every zone that changes.

What is the time difference between London and Tokyo?

Nine hours in January 2026: London is UTC+00:00 and Tokyo is UTC+09:00. In July it is eight hours, because London moves to UTC+01:00 while Tokyo stays on UTC+09:00. The time zone converter gives the difference for any two cities and any date.

Related pages

Sources

  • IANA Time Zone Database, via Node.js Intl (Intl.DateTimeFormat with timeZone) — offsets, daylight saving dates and local times in this article were worked out from the IANA database, not typed from memory.
  • TimeFlow's own time-zone data (data/cities.ts, data/dst.ts), computed from the IANA database.
  • Time zone rules and offsets: the IANA Time Zone Database, as applied by this site's tools. Offsets and daylight saving dates on this page are computed from the database for the dates stated.
  • TimeFlow's time zone converter, world clock and UTC converter — the live tools behind the numbers on this page.
  • Time-zone and daylight saving rules are set by governments and can change. For flights, deadlines and meetings that cannot move, confirm the time with an official source.