At the equator, the earth rotates 1,038 miles per hour, or 17¼ miles per minute. Going westward, to keep accurate time, one would have to set his watch back one minute every 17¼ miles traveled. Traveling eastward, one would set his watch ahead in the same proportions.

To avoid such difficulties, Earth was divided into twenty-four time zones by international agreement. A traveler, then, has only to set his watch ahead one hour as he enters a new time zone going east, or back one hour if he is traveling in a westerly direction.

Each of the twenty-four time zones is centered on the 15º meridian and all time within any zone is the same. The boundaries of the zones are irregular in many places to suit local convenience. Obviously, it would be impractical for a city to have two different times because it happened to straddle a time zone boundary.