The first national observance of Daylight Saving Time in the United States was in 1918, to save energy during World War I. This lasted seven months before being repealed. A similar, year-round, national observance went into effect in World War II, starting February 9, 1942, and ending September 30, 1945.

For the next two decades, there was no national rule regarding DST. Individual states, cities, and towns could start or end it at any point. This led to nationwide confusion, as the country became a patchwork of different observances. Finally, in 1966, Congress passed the Uniform Time Act, establishing DST as beginning on the last Sunday in April and ending on the last Sunday of October. No area was required to observe daylight saving time in the first place, but those that chose to do so were required to use the nationwide starting and ending dates.

Those starting and ending dates have changed over the years. From 1986 through 2006, DST ran from the first Sunday in April to the last Sunday in October. Beginning in 2007, the observance starts on the second Sunday in March and ends in the first Sunday in November, adding an additional month.

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