Daylight Saving Time, or DST, is the period of the year when clocks are moved one hour ahead. In the United States, this has the effect of creating more sunlit hours in the evening during months when the weather is the warmest. We advance our clocks ahead one hour at the beginning of DST, and move them back one hour ("spring forward, fall back") when we return to Standard Time (ST). This has the effect of moving one hour of daylight from the morning to the evening.DST was formally introduced in the United States in 1918. Today, most of the country and its territories observe DST. However, DST is not observed in Hawaii, American Samoa, Guam, Puerto Rico, the Virgin Islands and the state of Arizona (with the exception of the Navajo Indian Reservation, which does observe DST).Daylight Saving Time and time zones are regulated by the U. S. Department of Transportation, not by NIST. However, as an official timekeeper for the United States, NIST observes all rules regarding DST when it distributes time-of-day information to the public.
