Time measured by rotation of Earth is not uniform when compared with time kept by atomic clocks. It was not always so, and atomic clocks underwent changes before giving us accurate time. The first atomic clock, built at the U.S. National Bureau of Standards in 1949, was a maser with attached equipment. It was followed by advanced atomic clocks that provide high accuracy by allowing for microwave interrogation of atoms isolated from each other and from any exterior disturbance.

Atomic clocks are used as time standards for counting the passing seconds. In 1884, the Greenwich Mean Time or GMT was established as first global time scale and UTC, its atomic equivalent, was established as the official time for the world in January, 1972. The International Bureau of Weights and Measures, or BIPM, is the official keeper of atomic time for the world. In the U.S., the National Institute of Standards and Technology