In some of the power grids in the US, the clock-sync thing was something that the operations were required to maintain over a 24 hour period. One technique used to monitor this was a simple electric clock. Electric clocks have synchronous motors, meaning that changes in frequency of the supply would change the time. The target was to be sure that at, say, 6pm, that the clock would show exactly 6pm, even though it would vary a bit during the day.