Standard time in the U.S. and its territories is observed within nine time zones. Standard time within each time zone is an integral number of hours offset from a time scale called Coordinated Universal Time (abbreviated UTC), maintained by a large number of very precise "atomic clocks" at laboratories around the world, including the U.S. Naval Observatory.
|Atlantic standard time (AST)||subtract 4 hours from UTC|
|Eastern daylight time (EDT)||subtract 4 hours from UTC|
|Eastern standard time (EST)||subtract 5 hours from UTC|
|Central daylight time (CDT)||subtract 5 hours from UTC|
|Central standard time (CST)||subtract 6 hours from UTC|
|Mountain daylight time (MDT)||subtract 6 hours from UTC|
|Mountain standard time (MST)||subtract 7 hours from UTC|
|Pacific daylight time (PDT)||subtract 7 hours from UTC|
|Pacific standard time (PST)||subtract 8 hours from UTC|
|Alaska daylight time (AKDT)||subtract 8 hours from UTC|
|Alaska standard time (AKST)||subtract 9 hours from UTC|
|Hawaii-Aleutian daylight time (HDT)||subtract 9 hours from UTC|
|Hawaii-Aleutian standard time (HST)||subtract 10 hours from UTC|
|Samoa standard time (SST)||subtract 11 hours from UTC|
|Chamorro standard time (ChST)||add 10 hours to UTC|
Thus, 11:00 UTC is the same as 5:00 CST. When converting zone time to or from UTC, dates must be properly taken into account. For example, 10 March at 02:00 UTC is the same as 9 March at 21:00 EST. The table can also be used to determine the difference between the time observed in any two zones. For example, the table shows that eastern standard time is three hours "ahead" of Pacific standard time.
Time zones in the U.S. are defined in the U.S. Code, Title 15, Chapter 6, Subchapter IX - Standard Time. The Department of Transportation is responsible for time zone boundaries, which are also defined in the Code of Federal Regulations, Title 49, Subtitle A, Part 71 - Standard Time Zone Boundaries.
The World Time Zone Map outlines all of the time zones.
For more information on time, time scales, and accurate clocks, see the U.S. Naval Observatory Time Service Department pages. Related information can be found on the pages of the National Institute of Standards and Technology (NIST).
History of Standard Time in the U.S.
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), modified the beginning and ending dates of daylight time, and renamed the 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.
For very readable accounts of the history of standard time in the U.S., see:
Michael O'Malley: Keeping Watch, A History of American Time (Viking, 1990).
William H. Earle: "November 18, 1883: The Day That Noon Showed Up on Time", Smithsonian magazine, November 1983, pp. 193-208.
Ian R. Bartky and Elizabeth Harrison: "Standard and Daylight-saving Time", Scientific American, May 1979 (Vol. 240, No. 5), pp. 46-53.
Carlene E. Stephens: Inventing Standard Time (National Museum of American History, Smithsonian Institution, Washington, D.C., 1983)