Currently, daylight savings time begins in the United States on the first Sunday in April and ends on the last Sunday in October. On the first Sunday in April, clocks are set ahead one hour at 2:00 a.m. local standard time, which becomes 3:00 a.m. local daylight time. On the last Sunday in October, clocks are set back one hour at 2:00 a.m. local daylight time, which becomes 1:00 a.m. local standard time. These dates were recently modified with the passage of the Energy Policy Act of 2005. Starting in March 2007, daylight time in the United States will begin on the second Sunday in March and end on the first Sunday in November.
Not all places in the U.S. observe daylight time. In particular, Arizona, Hawaii, and most of Indiana do not use it.
Many other countries observe some form of "summer time", but they do not necessarily change their clocks on the same dates as the U.S.
Daylight time and time zones in the U.S. are defined in the U.S. Code, Title 15, Chapter 6, Subchapter IX - Standard Time.
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 Daylight saving time was repealed in 1919, but standard time in time zones remained in law.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. With the Energy Policy Act of 2005, the starting and ending dates have once again been shifted. Beginning in 2007, daylight time will start on the second Sunday in March and end on the first Sunday in November.
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.