The US Congress establishes time zones and approves daylight saving time.

The United States Congress is the legislative branch of the federal government of the United States. It is a bicameral legislature, including a lower body, the U.S. House of Representatives, and an upper body, the U.S. Senate. They both meet in the United States Capitol in Washington, D.C.

The Standard Time Act of 1918, also known as the Calder Act, was the first United States federal law implementing Standard time and Daylight saving time in the United States. It defined five time zones for the continental United States and authorized the Interstate Commerce Commission to define the limits of each time zone.

A time zone is an area which observes a uniform standard time for legal, commercial and social purposes. Time zones tend to follow the boundaries between countries and their subdivisions instead of strictly following longitude, because it is convenient for areas in frequent communication to keep the same time.

Daylight saving time (DST), also referred to as daylight savings time, daylight time, or summer time, is the practice of advancing clocks to make better use of the longer daylight available during summer so that darkness falls at a later clock time. The standard implementation of DST is to set clocks forward by one hour in spring or late winter, and to set clocks back by one hour to standard time in the autumn.