ABSTRACT
This chapter introduces the concepts of calendar metrology, which is the determination of civil time and date according to the modern calendar, and network chronometry, which is the determination of computer time relative to international standards as disseminated via computer network. It describes the methods conventionally used to establish civil time and date and the various timescales in use. The most important use of time and frequency metrology was for worldwide communications, navigation, and space sciences, for which time determination depends on astronomical observations of the Sun, Moon, and stars. In 1976, the Terrestrial Dynamic Time timescale was established as the proper time of a clock on the equator of Earth at mean sea level relative to International Atomic Time. A calendar is a mapping from epoch in some timescale to the times and dates used in everyday life.
