On March 19, 1918, U.S. Congress passed the Standard Time Act and established standard time and daylight savings time. In effect, Congress passed the Standard Time Act to do two main things: (i) create a uniform standard time within the United States and (ii) enact daylight savings time to save on the use of energy.

Standard time and daylights savings time were not new concepts. In fact, prior to the Standard Time Act, standard time had already been initiated by the US and Canadian railroads in 1883 to standardize train schedules. Prior to 1883, the time of day was simply a local matter where most cities and towns used a local clocks or solar time.

Savings time was also not a new concept. In 1784, Benjamin Franklin proposed daylight savings time to save on the use of candles. Franklin argued that millions of dollars could be saved by changing time to allow more daylight. However, it took over a century before his idea became popular. In World War I, Germany began observing daylight savings time in an effort to save on the cost of fuel and the rest of Europe followed shortly thereafter.

Congress finally adopted standard time and daylight savings by passing the Standard Time Act on March 19, 1918. However, the ideas remained so unpopular with the people that Congress even abolished daylight savings time after World War I.

In the early 1960s, the transportation industry argued for more nationwide consistency in time. Congress complied and in 1966 it passed the Uniform Time Act (P.L. 89-387). The Act mandated standard time within established time zones and provided for daylight savings time hour. However, states were allowed to exempt themselves from daylight savings time as long as the entire state did so.

Today, standard time in the US includes Eastern Standard Time, Central Standard Time, Mountain Standard Time, and West Coast Standard Time. Also, daylight savings time begins in the United States on the second Sunday in March and ends on the first Sunday in November. In particular, clocks are set ahead one hour at 2:00 a.m. local standard time on the second Sunday in March, which becomes 3:00 a.m. local daylight time. Then, on the first Sunday in November, clocks are set back one hour at 2:00 a.m. local daylight time, which becomes 1:00 a.m. local standard time. These dates were established by Congress in the Energy Policy Act of 2005 (Pub. L. no. 109-58, 119 Stat 594 (2005)).

Currently, the following states do not observe daylight savings time: most of Arizona, Hawaii, American Samoa, Puerto Rico, and the Virgin Islands.