Daylight saving time is a plan in which clocks are set one hour ahead of standard time for a certain period, so that darkness comes an hour later. The plan provides an additional hour of daylight in the evening. Most of the states of the United States observe daylight saving time. Wherever it is observed in the United States, it begins on the first Sunday in April and ends on the last Sunday in October. A state may decide to remain on standard time. States that lie in more than one time zone may use daylight time in one zone and not the other.

The chief purpose of daylight saving time is to save energy by reducing evening use of lighting. As a result, countries often first adopt daylight time during a war or other crisis. Britain, for example, went on daylight time or what it calls "Summer Time" during World War I (1914-1918). The United States adopted the plan in 1918 but repealed it in 1919. The United States also observed it from Feb. 9, 1942, to Sept. 30, 1945, due to World War II.

After the war, many states established some type of daylight saving time. Beginning in 1967, the entire nation went on daylight time from the last Sunday in April to the last Sunday in October. In the 1970's, a reduction in Arab oil exports caused a fuel shortage in the United States. To conserve energy, Congress enacted daylight time from Jan. 6 to Oct. 27, 1974, and from Feb. 23 to Oct. 26, 1975. Since 1987, daylight time has begun on the first Sunday in April.

Not all places in the U.S. observe daylight time. In particular, Hawaii and most of Arizona do not use it. Indiana adopted its use beginning in 2006.

In 2006, daylight time begins on April 2 and ends on October 29.
In 2007, daylight time begins on March 11 and ends on November 4. [New law goes into effect.]
In 2008, daylight time begins on March 9 and ends on November 2.
In 2009, daylight time begins on March 8 and ends on November 1.

Contributor: Joanne Petrie, J.D., Senior Attorney, Department of Transportation.