The development of highly accurate cesium-beam atomic clocks led to the redefinition of the second in 1967. This led to the recognition by scientists and technologists of the inadequacy of measuring time based on the erratic motion of the earth whose rate fluctuates by a few thousandths of a second a day. Attempts to couple GMT, based on the earth's motion, and the new definition of the second was highly unsatisfactory. A compromise time scale, Coordinated Universal Time (UTC), was devised and became effective on January 1, 1972.
UTC normally runs at the rate of cesium-beam atomic clocks. When the difference between this atomic time and one based on the Earth’s rotation approaches one second, a one-second adjustment (a "leap second") is made in UTC, thereby maintaining synchronization between arbitrary date-time and the Earth’s journey about its axis and the sun. Hence, the behavior of the Earth retains primacy for date-time, while actual elasped-time (laboratory time) is demarcated by the hyperfine atomic clock.
National Institute of Standards and Time (NIST) maintains cesium-beam clock systems. This data is coordinated with data from other atomic clocks located in more than 25 countries contribute data to the international UTC scale which is coordinated in Paris by the International Bureau of Weights and Measures (BIPM). The evolution in time measurement responsibility from observatories to the standards laboratories has accompanied the change from earth-time to atomic-time.