slave clock

slave clock (plural slave clocks)

1. A clock that is synchronised by a master clock.
   • 1960 September, M. J. Wilson, “Harlow Town station opened”, in Trains Illustrated, page 542:

     Special details of the new station are the crystal-regulated master clock in the concourse, which controls 11 "slave" clocks and will also control those to be installed in the Harlow Mill goods depot; [...].

• master clock