This guide covers What Is GMT? | 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a count — it is computed from the IANA Time Zone Database for the date stated, not copied from a table that may be out of date.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.

GMT — Greenwich Mean Time — is the mean solar time at the Royal Observatory in Greenwich, London, on the Prime Meridian (0 degrees longitude). It was the world's first global time standard, adopted at the International Meridian Conference in Washington in 1884.

GMT is a solar measurement: it is the average time the Sun crosses the Greenwich meridian over the course of a year. Because the Earth's orbit is elliptical and its axis is tilted, the Sun does not cross the meridian at exactly the same clock time every day — the difference, called the equation of time, can be up to 16 minutes. GMT averages that out.

GMT was replaced as the formal world standard by UTC (Coordinated Universal Time) in 1972, because atomic clocks are more precise than solar observations. But GMT lives on: Britain's winter time is still legally called GMT, the BBC still announces it, and the term is used worldwide to mean UTC+0.

In practice, when a schedule says 14:00 GMT, it means 14:00 UTC — the same moment. The difference between the two is under a second in everyday terms, and it only matters for astronomy and precision engineering.

In practice

GMT was born from the railway. Before standard time, every town in Britain kept its own solar time — Bristol was 10 minutes behind London — and that was fine until the trains arrived. A timetable cannot work if every station has a different time, so the Great Western Railway adopted 'railway time' in 1840, set to Greenwich. Parliament made it legal in 1880, and the rest of the world followed at the 1884 International Meridian Conference in Washington.

The Greenwich meridian itself was chosen for practical reasons, not scientific ones. Britain was the world's greatest maritime power, most ships already used Greenwich as their reference for navigation, and the Royal Observatory at Greenwich had been publishing nautical tables for a century. The conference voted for Greenwich over Paris, and the Prime Meridian has been at 0 degrees longitude ever since — though the modern meridian, defined by GPS, is actually about 100 metres east of the brass strip in the Observatory courtyard.

Today GMT survives mainly as a name. The formal world standard is UTC, kept by atomic clocks, and the time shown as GMT is in practice UTC+0. But the name is everywhere: Britain's winter time is legally GMT, the BBC announces it, and millions of people say 'GMT' when they mean UTC. The meridian, the observatory and the name are all still there — only the clocks have changed.

The numbers: what 12:00 UTC looks like around the world

The table below shows the local time in eight major cities when it is 12:00 UTC on 15 January 2026. The spread is the whole point of time zones: the same moment is a morning in New York, midday in London, evening in Mumbai and night in Tokyo.

City Local time when it is 12:00 UTC (15 January 2026) UTC offset
New York 7:00 am UTC−05:00
London 12:00 pm UTC+00:00
Paris 1:00 pm UTC+01:00
Dubai 4:00 pm UTC+04:00
Mumbai 5:30 pm UTC+05:30
Tokyo 9:00 pm UTC+09:00
Sydney 11:00 pm UTC+11:00
Los Angeles 4:00 am UTC−08:00

Two things stand out. First, the range: from 04:00 in Los Angeles to 23:00 in Sydney, the same instant covers almost the whole day. Second, the half hour: Mumbai is at 17:30, not 17:00 or 18:00, because India uses a half-hour offset (UTC+05:30). If you convert a time for Mumbai by rounding to the nearest hour, you will be half an hour out. The UTC converter shows the offset and the local time for any city on any date, and the time zone converter converts between any two cities directly.

The offsets change during the year for the cities that observe daylight saving. The same eight cities in January and in July:

City Offset in January 2026 Offset in July 2026 Changes?
New York UTC−05:00 UTC−04:00 Yes — one hour
London UTC+00:00 UTC+01:00 Yes — one hour
Paris UTC+01:00 UTC+02:00 Yes — one hour
Dubai UTC+04:00 UTC+04:00 No
Mumbai UTC+05:30 UTC+05:30 No
Tokyo UTC+09:00 UTC+09:00 No
Sydney UTC+11:00 UTC+10:00 Yes — one hour
Los Angeles UTC−08:00 UTC−07:00 Yes — one hour

New York, London, Paris, Sydney and Los Angeles all change by one hour between January and July; Dubai, Mumbai and Tokyo do not, because they do not observe daylight saving. That is why a conversion done in January can be wrong in July — and why the UTC converter asks for the date.

Key facts

  • UTC never observes daylight saving. It is the fixed reference that every other time zone is measured against.
  • The world uses 38 different UTC offsets, from UTC−12:00 to UTC+14:00 — a span of 26 hours.
  • Nepal is on UTC+05:45, the only 45-minute offset in regular use. India is on UTC+05:30.
  • The Prime Meridian is at 0 degrees longitude, through Greenwich, London.
  • A time written in UTC means the same thing everywhere on Earth, in January and in July.

Frequently asked questions

What does GMT stand for?

Greenwich Mean Time — the mean solar time at the Royal Observatory in Greenwich, London, on the Prime Meridian (0 degrees longitude).

When was GMT adopted?

It was adopted as the world's time standard at the International Meridian Conference in Washington in 1884, and Britain made it legal in 1880.

Is GMT still used?

Yes, as a name: Britain's winter time is legally GMT, the BBC announces it, and timetables print it. The formal world standard is now UTC, but GMT and UTC show the same time.

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.

Why was Greenwich chosen?

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.

What time is it in Tokyo when it is 12:00 UTC?

23:00 the same day — Tokyo is UTC+09:00 all year, so you add nine hours to 12:00 UTC. In New York it is 07:00 in winter (UTC−05:00) and 08:00 in summer (UTC−04:00). The UTC converter shows the time in any city for 12:00 UTC.

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.