Time zone

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A time zone is a region which shares the same standard time, rather than using the local solar time. Today, most of the world is divided into time zones, most 1 hour apart from adjacent zones, and approximately 15 degrees of longitude in width. The boundaries of time zones are designated by local convention or government action. Before time zones were instituted, local solar time would vary by one minute over distances of 6 km (4 miles) or less when moving east or west. With the development of accurate time-keeping and coordinated long-distance travel and communication, adjusting for changes in local solar time became burdensome.

History of time zones

In the 1840s a railway standard time for all of England, Scotland, and Wales evolved, replacing several "local time" systems. The Royal Observatory in Greenwich began transmitting time telegraphically in 1852 and by 1855 most of Britain used Greenwich time. Greenwich Mean Time (GMT) subsequently evolved as an important and well-recognized time reference for the world.

In 1830, the U.S. Navy established a depot, later to become the U.S. Naval Observatory (USNO), with the initial responsibility to serve as a storage site for marine chronometers and other navigation instruments and to "rate" (calibrate) the chronometers to assure accuracy for their use in celestial navigation. For accurate "rating," the depot had to make regular astronomical observations. It was not until December of 1854 that the Secretary of the Navy officially designated this growing institution as the "United States Naval Observatory and Hydrographic Office." Through all of the ensuing years, the USNO has retained timekeeping as one of its key functions.

Standard time in time zones was instituted in the U.S. and Canada by the railroads on 18 November 1883. Before then, time of day was a local matter, and most cities and towns used some form of local solar time, maintained by some well-known clock (for example, on a church steeple or in a jeweler's window). The new standard time system was not immediately embraced by all, however.

Use of standard time gradually increased because of its obvious practical advantages for communication and travel. Standard time in time zones was not established in U.S. law until the Act of March 19, 1918, sometimes called the Standard Time Act. The act also established daylight saving time, itself a contentious idea. Daylight saving time was repealed in 1919, but standard time in time zones remained in law, with the Interstate Commerce Commission (ICC) having the authority over time zone boundaries. Daylight time became a local matter. It was re-established nationally early in World War II, and was continuously observed until the end of the war. After the war its use varied among states and localities. The Uniform Time Act of 1966 provided standardization in the dates of beginning and end of daylight time in the U.S. but allowed for local exemptions from its observance. The act also continued the authority of the ICC over time zone boundaries. In subsequent years, Congress transferred the authority over time zones to the U.S. Department of Transportation (DOT), modifed (several times) the beginning date of daylight time, and renamed the three westernmost time zones.

Time zone boundaries have changed greatly since their original introduction and changes still occasionally occur. DOT issues press releases when these changes are made. Generally, time zone boundaries have tended to shift westward. Places on the eastern edge of a time zone can effectively move sunset an hour later (by the clock) by shifting to the time zone immediately to their east. If they do so, the boundary of that zone is locally shifted to the west; the accumulation of such changes results in the long-term westward trend. The process is not inexorable, however, since the late sunrises experienced by such places during the winter may be regarded as too undesirable. Furthermore, under the law, the principal standard for deciding on a time zone change is the "convenience of commerce". Proposed time zone changes have been both approved and rejected based on this criterion, although most such proposals have been accepted.


  • World Time Scales, in A Walk Through Time, the Evolution of Time Measurement Through The Ages, NIST Physics Laboratory, updated 2002.
  • History of Standard Time in the U.S., in What Are The U.S. Time Zones, U.S. Naval Observatory Astronomical Applications, updated 2006.