Time zones are based on the fact that the Earth moves 15 degrees longitude each hour. Since there are 24 hours in a day there are 24 standard time zones. (24 hours x 15º=360º)
Time zones are counted from the Prime Meridian (0º longitude). Each time zone is counted at 15 º intervals and extends 7½º either side of a central meridian. For example, Sydney, Australia lies in the 150ºE central meridian, and the time zone includes all locations between 142½ºE and 157½ºE.