What is Daylight Saving Time?

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). The transition from ST to DST has the effect of moving one hour of daylight from the morning to the evening. The transition from DST to ST effectively moves one hour of daylight from the evening to the morning.

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).

DST is now in effect for 238 days, or about 65% of the year, although Congress retained the right to revert to the prior law should the change prove unpopular or if energy savings are not significant. At present, daylight saving time in the United States

• begins at 2:00 a.m. on the second Sunday of March (at 2 a.m. the local time time skips ahead to 3 a.m. so there is one less hour in that day)
• ends at 2:00 a.m. on the first Sunday of November (at 2 a.m. the local time becomes 1 a.m. and that hour is repeated, so there is an extra hour in that day)