125 years ago today (on the 20th of October 1884)
41 delegates from 25 nations meat in Washington DC, USA
and agreed that the International Meridian for world time would use Greenwich Mean Time.
All the delegates voted for GMT except San Domingo.
(France and Brazil did not vote.)
72% of the world's shipping already depended on sea charts that used Greenwich as the Prime Meridian.
Since the 1960s, atomic clocks rather than astronomy have been keeping the world's time and have forced GMT to adapt. As atomic clocks are more accurate and the rotation of the Earth is irregular and slowing, mean atomic time and the real Earth time are slowly drifting apart. So leap seconds are sometimes added to GMT to realign it to the Earths orbit.
In Britain we use GMT, but during the summer we use British Summer Time. BST is GMT+1 hour meaning that 1:00pm BST is 12:00pm GMT.
BST was first used in 1916 when it was 80 minutes ahead of GMT and not 60 minutes as Dunlin mean time + 1 hour was used. Then on Sunday the 1st of October 1916 when the clocks went back to GMT from BST, Ireland then used GMT and not Dunlin mean time.
Duruing World Wor Two, Britian also used DBST. (WW2; 1940-1945, DBST; 1941 - 1947)
DBST or Double British Summer Time was used to increase the working times of daylight.
Since 1996 all clocks in Europe have changed on the same date from 'standard time' (winter time) to summer time 'Daylight Saving Time'
Daylight Saving Time starts on the last Sunday in March at 2:00am gmt / 3:00am bst.
Standard time starts on the last Sunday in October at 2:00am bst / 1:00am gmt.
If you go to Greenwich today (20/10/2009), don't use Greenwich mean time, as we're using daylight saving British summer time.