GMT stands for Greenwich Mean (or Meridian) Time and is the average time that the earth takes to rotate from noon-to-noon.

The use of Greenwich Meantime (GMT) began at 3pm on 25 September 1676, when two very accurate clocks were set in motion at the Royal Observatory at Greenwich, England. Greenwich Mean Time became standard for England; in 1884 it became standard for the world.

In the UK, GMT is the official time only during winter; during summer (March to October) British Summer Time is used.

At the Greenwich Observatory, the Shepherd clock was installed in about 1852. This was the first clock to show Greenwich Mean Time to the public.

Also on this day ....

The Great River Race - Thames, London

Over 200 different rowing boats race along the River Thames, this year from London Docklands to Ham House (Richmond), covering 22 miles of the River Thames. Around 300 crews from around the world take part, from those that enjoy fun and dress in fancy dress to the serious sports men and women who are in it to win it. Starts at 9am at London Docklands, first boat off at 11am.

25 September 1066: The Battle of Stamford Bridge

The Battle of Stamford Bridge took place at the village of Stamford Bridge, East Riding of Yorkshire in England on 25 September 1066, between an English army under King Harold Godwinson and an invading Norwegian force led by King Harald Hardrada of Norway and the English king's brother Tostig Godwinson.