Standard time is a worldwide system of uniform time zones. This system divides the world into 24 zones. Each zone is 15° longitude wide (see LONGITUDE). The difference in time between neighboring zones is exactly one hour. Within each zone, all clocks keep the same time, except for local variations.

Time zones. The local, or sun, time for any specific location depends on its longitude. There is a difference of 4 minutes for each degree of longitude, or a difference of an hour for every 15°. Under standard time, the time kept in each zone is that of the central meridian, or longitude line. The central meridians are those 15°, 30°, 45°, and so on, east or west of the prime, or Greenwich, meridian (see GREENWICH MERIDIAN). In theory, the zone boundaries should extend 71/2° on either side of the central meridian. In practice, the boundaries are irregular lines. This is to avoid inconvenient changes in time. For example, in the United States, zone boundaries often are located so that a state will lie entirely within one time zone. The Department of Transportation has the authority to establish limits for time zones in the United States. Time zones used in Canada have the same names as the time zones used in the United States.

The standard U.S. and Canadian time zones are--from east to west--Atlantic, Eastern, Central, Mountain, Pacific, Alaska, and Hawaii-Aleutian. Canada also has the Newfoundland Time Zone, but it is not a true standard time zone because it is only a half hour later than its neighboring zone to the west.

In summer, residents of most states advance clocks one hour to use daylight saving time. An act of Congress, which took effect in 1967, declared that daylight saving time must be used throughout a state or not at all. However, a 1972 amendment to the act allows states that lie in more than one time zone to use daylight time in one zone without using it in the other. See DAYLIGHT SAVING TIME.
Contributor: Joanne Petrie, J.D., Senior Attorney, Department of Transportation.