Daylight Saving Time Facts

Jennifer Vernon
for National Geographic News
Updated March 5, 2009

Daylight saving time in most of the United States starts this year in the early hours of March 8. The "spring forward" marks the second time the country has observed the switch in March rather than April since changes to the system were adopted in 2007.

Contrary to popular belief, no federal rule mandates that 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—will do nothing. Those locales never deviate from standard time within their particular time zones.

The recently revised federal law, first passed in 1918, now stipulates areas that observe daylight saving time must switch back to standard time at 2 a.m. on the first Sunday in November.

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

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."

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