The grid is managed to an exact _average_ frequency. In this way, it essentially acts as a very slow transfer standard for GPS. I own a clock with a 60Hz motor,

The grid is managed to an exact _average_ frequency. In this way, it essentially acts as a very slow transfer standard for GPS. I own a clock with a 60Hz motor, and while it can vary by anywhere from 5 to 30 seconds throughout a day, this error never accumulates and it recovers back towards zero after any excursions within a day or two.