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A worker adjusts the hands of a clock in preparation for Daylight Savings Time
A worker applies finishing touches at the Electric Time clock company in Massachusetts (file photo).

Photograph by Michael Fein, Bloomberg, Getty Images

National Geographic News

Updated March 12, 2010

When is the big daylight saving time (often called daylight savings time) switchover in spring 2010? And why do we spring forward in the first place?

Daylight saving time in most of the United States starts at 2 a.m., local time, on Sunday, March 14.

Contrary to popular belief, no federal rule mandates that U.S. states or territories observe daylight saving time.

Most U.S. residents set their clocks one hour forward in spring and one hour back in fall. But people in Hawaii and most of Arizona—along with the U.S. territories of Puerto Rico, American Samoa, and the Virgin Islands—do nothing. Those locales never deviate from standard time within their particular time zones.

The federal law first passed in 1918 and, thanks to a 2005 revision that went into practice in 2007, now stipulates that areas that observe daylight saving time must switch back to standard time at 2 a.m. on the first Sunday in November. (Read about the 2007 daylight savings time change and its purported energy savings.)

Likewise, the 2007 daylight saving time rule requires that regions that observe daylight saving time begin at the same time on the second Sunday in March.

(Related: "Leap Second Added to 2008—Tech Glitches to Come?")

The Dawn of Daylight Saving Time

The U.S. Naval Observatory in Washington, D.C., sets what is known as standard time in the country through its maintenance of atomic clocks. But the observatory has nothing to do with regulating daylight saving time.

Oversight of daylight saving time first resided with the Interstate Commerce Commission. In 1966 the U.S. Congress transferred that responsibility to the newly created Department of Transportation.

Congress ordered the transportation agency to "foster and promote widespread and uniform adoption and observance of the same standard of time within and throughout each such standard time zone."

So why is a transportation authority in charge of time laws? It all dates back to the heyday of railroads.

"In the early 19th century … localities set their own time," said Bill Mosley, a public affairs officer at the U.S. Department of Transportation.

"It was kind of a crazy quilt of time, time zones, and time usage. When the railroads came in, that necessitated more standardization of time so that railroad schedules could be published."

In 1883 the U.S. railroad industry established official time zones with a set standard time within each zone. Congress eventually came on board, signing the railroad time zone system into law in 1918.

The only federal regulatory agency in existence at that time happened to be the Interstate Commerce Commission, so Congress granted the agency authority over time zones and any future modifications that might be necessary.

Part of the 1918 law also legislated for the observance of daylight saving time nationwide. That section of the act was repealed the following year, and daylight saving time thereafter became a matter left up to local jurisdictions.

Daylight saving time was observed nationally again during World War II but was not uniformly practiced after the war's end.

Finally, in 1966, Congress passed the Uniform Time Act, which standardized the start and end dates for daylight saving time but allowed individual states to remain on standard time if their legislatures allowed it.

A 1972 amendment extended the option not to observe daylight saving time to areas on the border of two time zones but within the same U.S. state.

Before the move by Congress in 2005 to extend daylight saving time, the most recent modification occurred in 1986, when the start date was moved from the last Sunday in April to the first Sunday in April.

Daylight Saving Time = More Evening Daylight

The drive behind the switch was "to adjust daylight hours to when most people are awake and about," Mosley said.

Daylight saving time decreases the amount of daylight in the morning hours, so that more daylight is available during the evening.

Not everyone benefits from the daylight saving time change, Mosley conceded. Farmers and others who rise before dawn may have to operate in the dark a while longer before daybreak.

And some experts suggest that the extended hours implemented in 2007 to save energy won't actually do the trick. That's because people may use more electricity during the darker mornings, canceling out any savings from not using as much power at night.

Daylight saving time, however, can bring many benefits, Mosley said. Research has shown that more available daylight does decrease the number of traffic accidents, traffic fatalities, and incidences of crime.

Congress noted other advantages while updating legislation in 1986, including "more daylight outdoor playtime for the children and youth of our Nation, greater utilization of parks and recreation areas, expanded economic opportunity through extension of daylight hours to peak shopping hours and through extension of domestic office hours to periods of greater overlap with the European Economic Community."

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