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 Technologys NIST-F1 is an example of accuracy with neither gaining nor loosing a second. Atomic clock time is important for global positioning of satellites, various missiles, rocket programs, aviation programs, navy, power distribution, mobile and landline telephone systems, the Internet, GPS, and digital television. The movement of the earth causes random fluctuations in length of days and years, and the atomic clock has been able to manage the anomalies of time differences. A recent example is the leap second added due to slowing of the earths rotation on December 31, 2005.

The unique measurement capability and success of atomic clocks is such that time and frequency have far higher accuracy than any other physical quantity. NASA uses atomic clocks to provide reliable and consistent navigation for interplanetary space travel, where fractional disparities in clock tick rates can dramatically affect the navigation of spacecraft. Similarly, computers are coordinated with atomic clock time and sitting anywhere in the U.S., we can have access to precise minutes and seconds.