That is actually not true for the synchronized european power grid we are talking about here. The time that a clock that runs of the power grid is known as "Netzzeit" (grid time) and there is standards on how far that is allowed to deviate from the legal, public time.

There is websites such as https://www.swissgrid.ch/swissgrid/en/home/experts/topics/fr... that track and display the deviation.