Actually it's a couple of 'big connected grid's, but your point is valid. US frequency, in the long term, is very accurate.
Don't know if this is related, but I've noticed some networks do not have shows start/stop exactly on the hour/half-hour anymore. Sometimes a show runs 'long'. I've actually seen a couple of episodes claim to run from 8:00 PM to 9:02 PM.
Go figure. I remember when you could almost set your clock by when the first commercial break would come in. Remember 'Wild Wild West'? It was originally four commercial breaks and they would put a still image in each quadrant of the screen for each segment. Nowadays a show would go under with so little commercial space.
daestrom