A Grandmaster clock is presented. The Grandmaster clock has a GNSS interface receiving GNSS time sync data, a Real Time Clock (RTC) with a crystal oscillator, one or more communication interfaces exchanging data with clock correction sources, and a clock correction host. The clock correction host computes clock drift based on the GNSS time sync data and the data from clock correction sources, generating adjusted clock data, and writing the adjusted clock data to the RTC from different locations. The Grandmaster clock may be in a vehicle.