About Time: The story of GMT
23 March 2011 Last updated at 17:46 GMT
Theoretically, the world should be divided into 24 equal time zones, in which each zone differs from the last by one hour. But as the years have passed, the world has turned into a much more complicated place.
Time zones are now much more irregular and sometimes seem positively eccentric, affected as they are by political, geographical and social changes in the real world.
Add to that arguments over daylight saving, and you have a subject that has been the cause of furious debate around the world.
Time zones have not always existed, of course. Nick Higham explains how the expansion of the railways in Britain led to the establishment of Greenwich Mean Time.