Greenwich Mean Time (GMT)

Greenwich Mean Time (GMT) is the mean solar time at the Royal Observatory in Greenwich, London, reckoned from midnight. It is the time zone with offset UTC+0 and serves as the basis for the world's civil time system. In 1884, the International Meridian Conference selected the meridian at Greenwich as the world's prime meridian, making GMT the global time reference. Today, the United Kingdom uses GMT only during the winter months (late October to late March), switching to British Summer Time (BST, UTC+1) for summer. While GMT was historically the primary standard, it has been largely superseded by Coordinated Universal Time (UTC) for scientific purposes, though GMT remains the legal time in the UK during winter. For a live world clock, visit Time.now.

History of Greenwich Mean Time

The concept of mean time at Greenwich dates back to the 17th century, when the Royal Observatory was founded in 1675 to improve navigation at sea. Astronomers used the position of the sun to calculate local mean time, which was adopted by British mariners. By the 19th century, the need for a unified time standard grew with the expansion of railways. In 1847, the British railway companies adopted GMT as their standard, and in 1880, it became the legal time for all of Great Britain. The 1884 International Meridian Conference formalised the Greenwich meridian as the prime meridian, establishing GMT as the world's time reference. Today, the UK's timekeeping is governed by the GMT vs UTC relationship, with UTC providing atomic precision.

GMT vs BST: The UK's Seasonal Time Change

The United Kingdom observes a seasonal time change: from the last Sunday in March to the last Sunday in October, clocks are advanced by one hour to British Summer Time (BST, UTC+1). During BST, the UK is effectively one hour ahead of GMT. This practice, often called Daylight Saving Time, aims to make better use of daylight during the longer days of summer. The transition back to GMT occurs in autumn, when clocks are set back one hour. For a detailed comparison, see GMT vs BST. If you need to convert times between London and New York, our London To New York Time guide is helpful.

GMT Offset Table by Season

GMT and UTC: What's the Difference?

While GMT is based on the Earth's rotation, UTC is an atomic time scale that is more precise. In practice, GMT and UTC share the same offset (UTC+0) and are often used interchangeably for civil timekeeping. However, UTC is the international standard, while GMT is a time zone. The difference between the two is never more than 0.9 seconds, but leap seconds are added to UTC to keep it within 0.9 seconds of UT1 (a modern measure of mean solar time). For everyday use, they are effectively the same. Learn more on our GMT vs UTC page.

Frequently Asked Questions

What is Greenwich Mean Time (GMT)?

Greenwich Mean Time is the mean solar time at the Royal Observatory in Greenwich, London. It is the time zone with offset UTC+0 and is used as the basis for the world's time zones.

Is the UK always on GMT?

No, the UK uses GMT only during winter (from late October to late March). During summer, it switches to British Summer Time (BST, UTC+1).

What is the difference between GMT and BST?

BST is one hour ahead of GMT. The UK advances clocks by one hour in spring to BST and reverts to GMT in autumn.

Why is GMT used as a world time standard?

GMT was historically the world's time standard because the Greenwich meridian was chosen as the prime meridian at the 1884 International Meridian Conference. It is still widely used as a reference for time zones.

Is GMT the same as UTC?

For civil purposes, yes. UTC is the atomic time standard that replaced GMT, but they share the same offset (UTC+0). The difference is less than a second.

How do I convert GMT to my local time?

You can use online tools like time in London or TimeRight.Now for worldwide city times. For a multi-city world clock view, try CurrentTime.now. For European or US times, see European time and date and US time and date.