The US used to guarantee 5,184,000 cycles per day. That’s not, like, 1% or anything comparable, that’s zero (with temporary deviations). Any clock which referenced line frequency would be dead accurate over the long-term. There’s a system called Time Error Correction (TEC) which would do this (refer to WEQ-006 and the like). Obviously this error is measured relative to some reference clock.

I’m not sure what the current status is, but my understanding is that there are efforts to retire this system and allow the speed to drift more.