Current GMT Time

For roughly seven months of the year, "GMT" and "London time" are an hour apart — and almost everyone who writes "GMT" in the summer means the second one. Greenwich Mean Time is a fixed offset of UTC+0. The United Kingdom only uses it in winter; from the last Sunday in March to the last Sunday in October it runs on British Summer Time, UTC+1.

This is the source of most GMT confusion, and it is worse than an ordinary time-zone mix-up because the word looks so authoritative. "3pm GMT" reads like a precise, international statement. In July, from a British sender, it usually is not.

On a clock face, GMT and UTC are identical — both are offset zero. What separates them is where the time comes from.

GMT is astronomical. It is mean solar time on the Greenwich meridian: the average moment the Sun crosses that line, averaged because the real Sun runs early or late by up to about sixteen minutes over the year. Earth's rotation is the reference, and Earth's rotation is not perfectly regular.

UTC is atomic. It is derived from International Atomic Time, a weighted average of roughly 450 caesium and rubidium clocks in national laboratories worldwide, and it is kept within 0.9 seconds of Earth's actual rotation by the occasional insertion of a leap second. Twenty-seven have been added since 1972, the most recent at the end of 2016; the General Conference on Weights and Measures resolved in 2022 to stop adding them by 2035.

In practice: use UTC for anything technical — logs, APIs, flight plans, scientific records. Reserve GMT for British civil time in winter, and for the everyday sense in which people mean "the time in London, roughly." Our longer explainer covers the leap-second mechanics and the HTTP-header history in full: UTC vs GMT — what's the difference?

The railways forced the issue

Before the 1840s every British town kept its own solar time, and Bristol ran about ten minutes behind London. That was harmless until trains made it possible to outrun the discrepancy. The Great Western Railway adopted London time across its network in 1840, other companies followed, and "railway time" spread until Greenwich Mean Time became Britain's legal standard in 1880.

1884: a vote that ratified existing practice

At the International Meridian Conference in Washington, twenty-five nations chose Greenwich as the prime meridian. The argument was commercial rather than scientific: roughly two thirds of the world's shipping already navigated on Greenwich-based charts, and the alternative was asking the majority to redraw them. France abstained and continued to use the Paris meridian in official documents for decades afterwards.

The brass line is not quite zero

Visitors straddle the meridian strip at the Royal Observatory and photograph a GPS reading that stubbornly says about 0.0015° west. They are not wrong. The historic line marks the Airy Transit Circle, fixed by an instrument levelled against local gravity; modern positioning uses the IERS Reference Meridian, defined geocentrically from satellite data, which falls roughly 102 metres to the east. The line tourists queue for is a monument to the meridian, not the meridian itself.

Britain's experiments with its own clock

British Summer Time arrived in 1916 to save coal. During the Second World War the country went further, running British Double Summer Time — two hours ahead of GMT — through the summers. Between 1968 and 1971 Britain tried staying on UTC+1 all year; the experiment was abandoned after sustained complaints about pitch-dark winter mornings in Scotland and northern England, an argument that resurfaces every time permanent daylight saving is proposed.