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The history of standard time in the United States began November 18, 1883, when United States and Canadian railroads instituted standard time in time zones. 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. 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 Standard Time Act of 1918 of March 19, 1918, also known as the Calder Act (15 USC 260).〔(History of Daylight Time in the U.S. ), United States Naval Observatory〕 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 Daylight saving time was repealed in 1919, but standard time in time zones remained in law, with the Interstate Commerce Commission (ICC) having the authority over time zone boundaries.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, the United States Congress transferred the authority over time zones to the U.S. Department of Transportation (DOT), modified (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. ==War Time 1918 and 1942== Daylight saving time was established by the Standard Time Act of 1918. The Act was intended to save electricity for seven months of the year, during World War I.〔(A Time-Change Timeline ), National Public Radio, March 8, 2007〕 DST was repealed in 1919 over a Presidential veto,〔An Act For the repeal of the daylight-saving law, 〕 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. During World War II, Congress enacted the War Time Act (56 Stat. 9) on January 20, 1942. Year-round DST was reinstated in the United States on February 9, 1942, again as a wartime measure to conserve energy resources.〔 (24/7: A Resource Guide to the Law of Time Standards" ), June 3, 2002〕 This remained in effect until after the end of the war. The Amendment to the War Time Act (59 Stat. 537), enacted September 25, 1945, ended DST as of September 30, 1945. During this period, the official designation War Time was used for year-round DST. For example, Eastern War Time (EWT) would be the equivalent of Eastern Daylight Time during this period. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「History of time in the United States」の詳細全文を読む スポンサード リンク
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