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 a.m. local standard time (which becomes 3 a.m. local daylight time). On the first Sunday in November, this year on Nov. 6, clocks are set back one hour at 2 a.m. local daylight time (which becomes 1 a.m. local standard time).
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.
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 Daylight saving time was repealed in 1919, but standard time in time zones remained in law.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 Feb. 9, 1942, to Sept. 30, 1945. After the war its use varied among states and localities.
—Courtesy of the U.S. Naval Operations Astronomical Applications Department