Standard Time begins each year at 2:00 a.m. (local time) on the first Sunday of November. Move your clocks back one hour at the resumption of Standard Time.
Daylight Saving Time (DST) begins each year at 2:00 a.m. (local time) on the second Sunday in March in most of the United States and its territories. Clocks must be moved ahead one hour when DST goes into effect. DST is not observed in Hawaii, American Samoa, Guam, Puerto Rico, the Virgin Islands and the state of Arizona (with the exception of the Navajo Indian Reservation, which does observe DST).
History of Daylight Time in the U.S.
Although standard time in time zones was instituted in the U.S. and Canada by the railroads in 1883, it 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, a contentious idea then. Daylight saving time was repealed in 1919, but standard time in time zones remained in law. Daylight time became a local matter. It was re-established nationally early in World War II, and was continuously observed from 9 February 1942 to 20 September 1945. 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 provided that daylight time begin on the last Sunday in April and end on the last Sunday in October, with the changeover to occur at 2 a.m. local time.
During the "energy crisis" years, Congress enacted earlier starting dates for daylight time. In 1974, daylight time began on 6 January and in 1975 it began on 23 February. After those two years the starting date reverted back to the last Sunday in April. In 1986, a law was passed permanently shifting the starting date of daylight time to the first Sunday in April, beginning in 1987. The ending date of daylight time has not been subject to such changes, and has remained the last Sunday in October.
For a very readable account of the history of standard and daylight time in the U.S., see
Ian R. Bartky and Elizabeth Harrison: "Standard and Daylight-saving Time", Scientific American, May 1979 (Vol. 240, No. 5), pp. 46-53.