From the U.S. Naval Observatory and the National Institute of Standards and Technology:
Daylight Saving Time (DST) begins each year at 2 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).
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.
In 2008, DST is from 2:00 a.m. (local time) on March 9th until 2:00 a.m. (local time) on November 2nd.
In 2009, DST is from 2:00 a.m. (local time) on March 8th until 2:00 a.m. (local time) on November 1st.
On August 8, 2005, President Bush signed the Energy Policy Act of 2005, which included the changes in Daylight Saving Time described above, effective in 2007. Prior to 2007, DST began at 2:00 a.m. (local time) on the first Sunday in April, and ended at 2:00 a.m. (local time) on the last Sunday in October. The new rules for DST beginning in 2007 mean an extra four or five weeks of DST each year. There will now be a total of 238 days of DST, compared to a total of 210 days of DST in 2006 under the previous rules, and the U. S. will remain on DST for about 65% of the year.
NIST broadcasts from radio station WWVB are used to synchronize many millions of wall clocks, watches, clock radios, and other timekeeping devices. The NIST radio broadcasts include information on whether or not DST is in effect, so radio controlled clocks will adjust whenever we transition between DST and ST.
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 Daylight saving time was repealed in 1919, but standard time in time zones remained in law.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.