KE7GKP:
Line frequency is not that accurate
For the purposes of long-term time accuracy, power mains frequency is just as accurate as the NITS Atomic Clock. Because all the utility companies maintain the grid frequency against WWV.
Yeah that 'NITS' atomic clock isn't all it's hyped up to be 
To satisfy my curiosity (which you're obviously under no obligation to do...), can you point to something that documents this? Oh wait, here it is: Utility frequency - Wikipedia