Abstract: This paper introduces the development of the lunar standard time and its definition based on general relativity and the simultaneity principle in four-dimensional spacetime. A three-step progressive approach is proposed to realize a coordinated lunar time. The primary step, with standard time on the earth ground adopted for traceability, applies to those without clocks deployed on lunar ground or in orbit thus unable to represent time unit on the moon. The intermediate step, with proper time of local clocks converted into coordinated time and an independent lunar standard time provided for time service, applies to those with at least 1 clock on the lunar ground representing SI second and are authorized as lunar local time reference by metrological administration. The advanced step, with time standard traced to an agreed convention by decentralized time keeping and broadcasting, time reference is not directly authorized anymore but replaced with different time keeping apparatus ID. This step applies to those with at least 4 time keeping devices deployed in lunar space or on the lunar ground. Users could obtain local time and position information from broadcast timestamps, and this mechanism is basically consistent with the existing Global Navigation Satellite System (GNSS) theories. Lunar standard time operates independently of Earth standard time. To meet the demand of users on both celestial bodies, timekeeping devices shall continuously monitor and release coordinate time comparison tables as well as terrestrial and lunar calendars. Those timekeeping methods in the three steps mentioned above provide theoretical reference for metrological specifications concerning the “Definition of Lunar Standard Time”.

Key words: moon time, timekeeping device, time service device, decentralized timekeeping, four-dimensional spacetime simultaneity