daylight saving time or summer time

Benjamin Franklin seems to have been the first to propose transferring hours of early morning daylight, which is little used by a modern, late-rising, industrial society, to the evening. Currently the chief benefit is savings in electric power used for lighting.

Daylight saving time is mainly beneficial to countries in the earth's middle latitudes. Nearer the poles, the summer day is so long that the evening is daylit anyway. In the tropics, there is little seasonal variation in the length of the day.


Germany adopted daylight saving in 1915 as a wartime conservation effort, followed by Great Britain in April 1916 (where it was at first called Willett Time, after William Willett, a proponent). In World War II, Britain advanced the clock one hour during the winter (bst, for British Summer Time) and two hours during the summer (dbst, for Double British Summer Time), first for five months, then seven, and finally year round between 1941 and 1944. From 1968 to 1971, Britain tried keeping bst (now standing for British Standard Time) year round, so that it would be the same time in Britain as in Western Europe. It was abandoned because the west and north complained that winter mornings were too dark.

At present, summer time in the European Union begins at 1:00 UT on the last Sunday in March, and ends at 1:00 UT on the last Sunday in October.

United States

The United States adopted daylight savings as an energy conservation measure during both world wars. In 1917, Congress called for adding one hour from the last Sunday in March to the last Sunday in October. In 1922, the period was changed to the Sunday following the third Saturday in April (unless it was Easter) to the Sunday following the third Saturday in September. In 1919, daylight saving was repealed, largely at the insistence of farmers. From 9 February 1942 to 30 September 1945, all clocks were advanced one hour year round; there was no summer addition. In 1966 the Uniform Time Act established daylight saving time nationwide as of 1 April 1967. In principle a state can opt out only if the entire state does so, but there are exemptions. See U.S. time zones and United States laws relating to Daylight Savings time.

Clocks are advanced one hour from the second Sunday of March to the first Sunday of November. (In 1986, the beginning date was changed to the first Sunday in April from the last Sunday in April¹. In 2007, the dates were changed again, to the second Sunday of March, and from the last Sunday of October to the first Sunday of November.²) The changeover is made at 2 a.m.. “Spring ahead, fall behind” has become perhaps the best-known mnemonic in the United States.

Thanks to Cornell University, current federal law regarding daylight saving may be accessed at


1. Public Law 99-359, subsection (a).

2. Public Law 109-58, title I, � 110(a), (b), Aug. 5, 2005, 119 Stat. 615. 

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