In the United States, daylight saving time begins on the first Sunday in April and ends on the last Sunday in October. On the first Sunday in April, clocks are set ahead one hour (spring ahead) at 2:00 a.m. local standard time, which becomes 3:00 a.m. local daylight saving time. On the last Sunday in October, clocks are set back one hour at 2:00 a.m. local daylight saving time, which becomes 1:00 a.m. local standard time.

Not all places in the U.S. observe daylight saving time. In particular, Arizona, Hawaii, and most of Indiana do not use it.

Many other countries observe some form of "summer time", but they do not necessarily change their clocks on the same dates as the U.S.

Daylight saving time and time zones in the U.S. are defined in the U.S. Code, Title 15, Chapter 6, Subchapter IX -

History of Daylight Time in the US:

Standard time in time zones was instituted in the U.S. and Canada by the railroads in 1883, but it was not established in U.S. law until the Act of March 19, 1918 (sometimes called the Standard Time Act). The act also established daylight saving time, a contentious idea then. Daylight saving time was repealed in 1919, but standard time in time zones remained in law. Daylight saving time became a local matter. It was re-established nationally during the early part World War II, and was continuously observed from February 9, 1942 to September 20, 1945. After the war its use varied among states and localities. The Uniform Time Act of 1966 provided standardization in the dates of beginning and end of daylight saving time in the U.S. but allowed for local exemptions from its observance. The act provided that daylight saving time begin on the last Sunday in April and end on the last Sunday in October, with the changeover to occur at 2 a.m. local time.