The Earth's surface was divided into 24 equal segments, with the 360 degrees divided into 15 degree slices. The division started at the meridian crossing Greenwich, England (the prime meridian) and progressed from there. Each 15 degree change in longitude causes about a 1 hour change in sunrise time at the equator. Thus, these meridians were used to define time-zones. However, there are many places where time-zone borders were modified to allow related areas that straddle one of the potential time-zone borders to remain in a single time-zone.

For a complete map of time zones see http://www.worldtimezone.com/ . Note also that at 180 degrees around from the prime meridian is the International Date Line.

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