Five dates anchor the story of Greenwich time: 1675, 1847, 1880, 1884 and 1972. Royal Museums Greenwich dates the Royal Observatory to 1675, the railways’ adoption of Greenwich time across Great Britain to December 1847, its legal status to 1880, the vote for the world’s prime meridian to 1884 and the end of Greenwich Mean Time’s run as the international standard to 1972. This guide walks through those dates, then three easily confused terms: GMT, UTC and BST.
1675 to 1880: from observatory time to legal time
Royal Museums Greenwich (RMG) says the Royal Observatory was founded by Charles II in 1675 to find a way to measure longitude at sea. It defines Greenwich Mean Time as the yearly average of the time each day when the Sun crosses the Prime Meridian at the Royal Observatory, and says that today GMT is reckoned from one midnight to the next.
RMG says that until the mid-19th century almost every town kept its own local time. It gives Bristol Mean Time as 10 minutes behind GMT and Cardiff Mean Time as 13 minutes behind. GMT was adopted across Great Britain by the Railway Clearing House in December 1847 and became known as “Railway Time”. By the mid-1850s almost all public clocks in Britain were set to it, and it became Britain’s legal standard time in 1880.
1884: the vote in Washington
RMG says that in 1884 representatives of different nations gathered in Washington D.C. to choose a single meridian, and that after weeks of discussion delegates voted for the Greenwich Meridian. The cross-hairs of the Airy Transit Circle’s eyepiece defined zero degrees longitude. The delegates also recommended a global time zone system based on GMT.
RMG gives its reasons in two ways on two pages. One page says the United States had already chosen Greenwich as the basis of its time zones and that 72% of the world’s commerce depended on sea-charts using Greenwich as the Prime Meridian. Another says Greenwich offered the least disruption because nearly two-thirds of the world’s ships were already using charts based on it. The two are worded differently, one about commerce and one about ships, and are reported here as worded.
GMT, UTC and BST: three labels, three jobs
RMG says GMT was the international standard of civil time from 1884 until 1972 and has since been replaced by Coordinated Universal Time (UTC). It also says GMT is still the legal time in Britain in the winter. A separate RMG page says that civil time in the UK is Coordinated Universal Time (UTC), described as the modern equivalent of Greenwich Mean Time (GMT) and never more than half a second different from it, and that in summer clocks move to British Summer Time (BST), one hour in advance of GMT.
GOV.UK gives the clock changes: forward 1 hour at 1am on the last Sunday in March, back 1 hour at 2am on the last Sunday in October. The period when clocks are 1 hour ahead is BST, and when the clocks go back the UK is on GMT. Its page, last updated 24 November 2025, lists these dates:
| Year | Clocks go forward | Clocks go back |
|---|---|---|
| 2026 | 29 March | 25 October |
| 2027 | 28 March | 31 October |
| 2028 | 26 March | 29 October |
The traps follow from the wording. GMT is not a synonym for UK clock time all year, because GOV.UK says the UK is on GMT when the clocks go back. RMG says GMT was the international standard until 1972 and has since been replaced by UTC.
Is the Greenwich line still the Prime Meridian?
RMG says the historic line is no longer the internationally agreed one. Between 1984 and 1988 new coordinate systems based on satellite data and other measurements were adopted, requiring a prime meridian that defines a plane passing through the centre of the Earth. The current agreed meridian, the IERS Reference Meridian, passes 102.5 m east of the historic Prime Meridian, which is why, RMG says, a phone standing on the historic line will not show exactly zero degrees longitude.
RMG also gives numbers for the line itself: 20,000 km from pole to pole, passing through eight countries (the UK, France, Spain, Algeria, Mali, Burkina Faso, Togo and Ghana), crossing three continents, with Antarctica the only landmass it crosses in the southern hemisphere. For nearly two thirds of its length it passes over the sea. At night a laser beam marks it at the Observatory.
The clock that ignores summer time
The Shepherd Gate clock at the Observatory gates is described by RMG as the first clock ever to show Greenwich Mean Time directly to the public. Its dial has 24 hours, so at noon the hour hand points straight down. RMG says it originally showed astronomical time, counting the day from noon, was later changed to count from midnight, and continues to show GMT without adjustment for BST.
The bottom line
On the RMG and GOV.UK pages read, the safe dates are 1675 for the Observatory, December 1847 for the Railway Clearing House, 1880 for legal standard time in Britain, 1884 for the Washington vote and 1972 as the end of GMT’s run as the international standard. For the UK, RMG says GMT is the legal winter time and UTC is its modern equivalent within half a second, while GOV.UK says the UK is on GMT after the clocks go back and on BST while they are an hour ahead. RMG’s two pages word the 1884 reasons differently, so the figures should not be merged, and RMG says the agreed prime meridian is now the IERS Reference Meridian, 102.5 m east of the historic line.
Sources
- What is Greenwich Mean Time (GMT) – and why does it matter?, Royal Museums Greenwich, page undated, read 5 November 2026 (rmg.co.uk)
- What is the Prime Meridian, and why is it in Greenwich?, Royal Museums Greenwich, page undated, read 5 November 2026 (rmg.co.uk)
- Which time is it?, Royal Museums Greenwich, page undated, read 5 November 2026 (rmg.co.uk)
- When do the clocks change?, GOV.UK, last updated 24 November 2025, read 5 November 2026 (gov.uk)
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