Time Zone

Time zones are areas of the earth that have the same local time. Ideally, time zones are centered on lines of a longitude that are a multiple of 15║ from the Prime Meridian. This spacing would provide a total of 24 time zones, one for each hour in a day.

In practice, time zone boundaries are not perfect North-South lines along a meridian. Instead, they zig and zag, following country and state boundaries, and even geological or topographic features of the earth. 

There are also more than 24 different time zones. Some of the irregular time zones are not a whole number of hours away from GMT. Others differ from their brethren by having different rules for the observation of Daylight Saving Time.