By act of Congress, civil clocks in most areas of the United States are adjusted ahead one hour in the summer months (known as daylight time) and returned back one hour in the winter months (known as standard time). The dates marking the beginning and end of daylight time have changed as Congress has passed new statutes. As of 2007, daylight time begins in the United States on the second Sunday in March and ends on the first Sunday in November. On the second Sunday in March, 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 first Sunday in November, 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 established by Congress in the Energy Policy Act of 2005, Pub. L. no. 109-58, 119 Stat 594 (2005).
Not all places in the U.S. observe daylight time. In particular, Hawaii and most of Arizona do not use it. The most recent change to local daylight time policy was in 2006, when Indiana adopted the use of daylight time state-wide.
Many countries observe some form of "summer time." Most of the northern hemisphere countries which observe daylight time are located in Europe and North America. A handful of nations in the southern hemisphere observe summer time, but their starting and ending periods are reversed since summer in the southern hemisphere occurs during the northern hemisphere's winter months. The dates when daylight time changes are made are various and not determined by any international agreements.
Daylight time and time zones in the U.S. are defined in the U.S. Code, Title 15, Chapter 6, Subchapter IX - Standard Time.
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 Daylight saving time was repealed in 1919, but standard time in time zones remained in law.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 30 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 to the last Sunday in April. In 1986, a law was passed that shifted the starting date of daylight time to the first Sunday in April, beginning in 1987. The ending date of daylight time was not subject to such changes, and remained the last Sunday in October. The Energy Policy Act of 2005 changed both the starting and ending dates. Beginning in 2007, daylight time starts on the second Sunday in March and ends 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.