There used to be a local hydro generation that fed the local city, they had 2 clocks on the wall, one on the local grid and one on the national one - some time

There used to be a local hydro generation that fed the local city, they had 2 clocks on the wall, one on the local grid and one on the national one - some time late at night they'd speed up/slow down the local grid to keep the clocks in sync