<P> 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 . </P> <P> 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 . </P> <P> 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 The Act was intended to save electricity for seven months of the year, during World War I. DST was repealed in 1919 over a Presidential veto, but standard time in time zones remained in law, with the Interstate Commerce Commission (ICC) having the authority over time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 . </P> <P> 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 . 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 . </P>

When did the us begin using daylight savings time