Monday, March 7, 2011 6:23 am

At 2 a.m. on March 13, 2011, groggy Americans will turn their clocks forward one hour, marking the beginning of Daylight Saving Time (DST).

The federal law that established "daylight time" in the United States does not require any area to observe daylight saving time. But if a state chooses to observe DST, it must follow the starting and ending dates set by the law.

From 1986 to 2006 this was the first Sunday in April to the last Sunday in October, but starting in 2007, it is observed from the second Sunday in March to the first Sunday in November, adding about a month to daylight saving time.

Daylight saving time (DST), is time observed when clocks and other timepieces are set ahead so that the sun will rise and set later in the day as measured by civil time. The amount of daylight on a given day of the year at a given latitude is fixed, but over the year the hours of sunrise and sunset vary from day to day.

During the summer months, the sun rises earlier and sets later and there are more hours of daylight. If clocks and other timepieces are set ahead in the spring by some amount (usually one hour), the sun will rise and set later in the day as measured by those clocks.

This provides more usable hours of daylight for activities that occur in the afternoon and evening, such as outdoor recreation. Daylight saving time can also be a means of conserving electrical and other forms of energy. In the fall, as the period of daylight grows shorter, clocks are set back to correspond to standard time.

Benjamin Franklin, when serving as U.S. minister to France, wrote an article recommending earlier opening and closing of shops to save the cost of lighting. In England, William Willett in 1907 began to urge the adoption of daylight saving time.

During World War I the plan was adopted in England, Germany, France, and many other countries. In the United States, Robert Garland of Pittsburgh was a leading influence in securing the introduction and passage of a law (signed by President Wilson on Mar. 31, 1918) establishing daylight saving time in the United States.

After World War I the law was repealed (1919). In World War II, however, national daylight saving time was reestablished by law on a year-round basis. National year-round daylight saving time was adopted as a fuel-saving measure during the energy crisis of the winter of 1973–74.

In late 1974, standard time was reinstituted for the winter period. In 1987 federal legislation fixed the period of daylight saving time in the United States as the first Sunday (previously the last Sunday) in April to the last Sunday in October; it was expanded in 2005 (effective 2007) to extend from the second Sunday in March to the first Sunday in November.

Arizona and Hawaii do not use daylight saving time.