On Sunday, March 14, 2010 at 2 a.m., Daylight Saving Time begins in the United States.
The phrase "Spring forward, Fall back" helps people remember how Daylight Saving Time affects their clocks. At 2 a.m. on the second Sunday in March, we set our clocks forward one hour ahead of Standard Time ("Spring forward"). We "Fall back" at 2 a.m. on the first Sunday in November by setting our clock back one hour and thus returning to Standard Time.
The change to Daylight Saving Time allows us to use less energy in lighting our homes by taking advantage of the longer and later daylight hours. During the eight-month period of Daylight Saving Time, the names of time in each of the time zones in the U.S. (map) change as well. Eastern Standard Time (EST) becomes Eastern Daylight Time, Central Standard Time (CST) becomes Central Daylight Time (CDT), Mountain Standard Time (MST) becomes Mountain Daylight Time (MDT), Pacific Standard Time becomes Pacific Daylight Time (PDT), and so forth.
Daylight Saving Time was instituted in the United States during World War I in order to save energy for war production by taking advantage of the later hours of daylight between April and October. During World War II the federal government again required the states to observe the time change. Between the wars and after World War II, states and communities chose whether or not to observe Daylight Saving Time. In 1966, Congress passed the Uniform Time Act, which standardized the length of Daylight Saving Time.
Daylight Saving Time is four weeks longer since 2007 due to the passage of the Energy Policy Act in 2005. The Act extended Daylight Saving Time by four weeks from the second Sunday of March to the first Sunday of November, with the hope that it would save 10,000 barrels of oil each day through reduced use of power by businesses during daylight hours. Unfortunately, it is exceedingly difficult to determine energy savings from Daylight Saving Time and based on a variety of factors, it is possible that little or no energy is saved by Daylight Saving Time.
Arizona (except some Indian Reservations), Hawaii, Puerto Rico, the U.S. Virgin Islands, and American Samoa have chosen not to observe Daylight Saving Time. This choice does make sense for the areas closer to the equator because the days are more consistent in length throughout the year.
Daylight Saving Time Around the World
Other parts of the world observe Daylight Saving Time as well. While European nations have been taking advantage of the time change for decades, in 1996 the European Union (EU) standardized a EU-wide European Summer Time. This EU version of Daylight Saving Time runs from the last Sunday in March through the last Sunday in October.
In the southern hemisphere, where Summer comes in December, Daylight Saving Time is observed from October to March. Equatorial and tropical countries (lower latitudes) don't observe Daylight Saving Time since the daylight hours are similar during every season; so there's no advantage to moving clocks forward during the Summer.