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.
China is a huge country yet uses just one time zone. It means that there are dramatic differences in the time the sun rises, from one side of the country to the other.
Martin Patience explains.
