- An internationally agreed time standard based on time kept by atomic clocks, and which represents the local time at the 0° meridian, which passes through Greenwich, England. When utilizing World Time, each location on the planet has the same time, which is expressed utilizing a 24 hour clock, with a Z frequently appended. For example, 2100Z indicates 9 PM. Since the earth's rotation is gradually slowing, an extra second is added approximately once a year. It is based on International Atomic Time. Also called Universal Coordinated Time, Universal Time Coordinated, Universal Time, Coordinated Universal Time, Zulu Time, or Z time.