The time zone boundaries and DST observance are regulated by the Department of Transportation, but no single map of those existed until the agency announced intentions to make one in September 2022.

[1] Official and highly precise timekeeping services (clocks) are provided by two federal agencies: the National Institute of Standards and Technology (NIST) (an agency of the Department of Commerce); and the United States Naval Observatory (USNO).

[2] American railroads maintained many different time zones during the late 19th century.

Time calculation became a serious problem for people traveling by train (sometimes hundreds of miles in a day), according to the Library of Congress.

[3] Operators of the new railroad lines needed a new time plan that would offer a uniform train schedule for departures and arrivals.

The U.S. time-zone system grew from this, in which all zones referred back to GMT on the prime meridian.

Standard time zones in the United States and other regions are currently defined at the federal level by statute 15 U.S.C. § 260.

[7] As of August 9, 2007, the standard time zones are defined in terms of hourly offsets from UTC.

Clocks are set ahead one hour at 2:00 a.m. on the following start dates and set back one hour at 2:00 a.m. on the corresponding end dates: In response to the Uniform Time Act of 1966, each state has officially chosen to apply one of two rules over its entire territory: The Energy Policy Act of 2005 extended daylight saving time for an additional month beginning in 2007.