time signal 
noun
An audible signal indicating the exact time of day, especially one broadcast by radio at certain times.The BBC first used the six pips as a broadcast time signal - in recent years the sixth pip has been elongated for emphasis....
- It was wound solemnly each Sunday morning, checked against the BBC time signal, adjusted, and the glass cover snapped gently back for another week.
- Every night, the clock automatically checks the radio-delivered official time signal and makes whatever adjustments are necessary to stay synchronized.