We used to get power from a local hydro project, at the power station they used to (ie prior to ubiquitous computers and all that) have two clocks on the wall,

We used to get power from a local hydro project, at the power station they used to (ie prior to ubiquitous computers and all that) have two clocks on the wall, one on the local grid and one on the national grid, periodically they would manually speed things up locally to keep the clocks close together as possibly