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Daylight Saving Time is a system of time in which clocks are set ahead of standard time during the summer months. In most cases, clocks are advanced one hour. Daylight saving time is usually in force from the end of April to the end of October. For the southern hemisphere it is reversed.
Daylight saving time was first used in Great Britain in 1916, when it was put into effect to save coal used for generating electricity. In 1918 an act of Congress put daylight saving time into effect throughout the United States. The act was repealed in 1919, but many cities continued to use daylight saving time.
During World War II, daylight saving time was again put into effect throughout the United States. In 1967, the period during which daylight saving time is observed was standardized in the United States. In late 1973, most of the United States went on temporary year-round daylight saving time in order to conserve energy. In late 1974, standard time was reinstated from the end of October to the end of February. In April 1975, the United States returned to the observance of regular daylight saving time.