Global Navigation Satellite System(GNSS)is based on precision time measurement, an uniform internal time reference is necessary to ensure all parts run synchronously, and to realize navigation, position and timing. All GNSS system have precision time reference systems, GNSST, at the same time, it is necessary that GNSST should be traced to the internal legal time standard, UTC(k), and then traced to international standard time, UTC, to ensure global time synchronous and uniform and realize the function of time service. The Coordinated Universal Time (UTC) and its realization, GNSS time references and their traceability, and GNSS time compatibility and interoperability are introduced briefly in this paper.