[astronomy] The official timekeeping system of the world’s nations since 1972. It refers local time throughout the world to time at the prime meridian, and is based on atomic clocks, but is periodically artificially adjusted so as to always remain within 0.9 seconds of universal time. The adjustment is made by the addition of leap seconds to the course of atomic time. Coordinated universal time is abbreviated “UTC.” (The abbreviation UTC does not represent the word order of “coordinated universal time” in either English or French. It is an extension of the “UT*” pattern established for versions of universal time.