Which U.S. law established standard time zones in 1918?

The Standard Time Act established standard time zones in the United States in 1918.

Also called the Calder Act, the law was approved on March 19, 1918, during World War I. It defined five standard time zones for the continental United States and authorized the Interstate Commerce Commission to set their boundaries. Railroads had already coordinated standard time across much of the country in 1883, but the practice had not previously been established in federal statute.

The act also introduced daylight saving time as a wartime energy measure. That provision proved unpopular and was repealed in 1919, although standard time zones remained part of federal law.

The Uniform Time Act of 1966 is a common wrong answer because it later standardized daylight-saving rules and expanded federal control. It did not establish the original U.S. time-zone system. “Railway time” describes the earlier railroad practice, not the name of the 1918 law.