Grids ensure the cycle count is correct over the day. There will be some drift from spikes and peak loads, but they adjust the frequency to compensate. They have atomic clocks, for the EU grid it's in Switzerland, to track the cycle count.

UK and EU grids have websites letting you look at frequency history.

There's still an awful lot of infrastructure keyed off having an accurate synchronous time, including billing systems, traffic lights, street lights...