I don't know how they do it, but they certainly know when suppertime is.
And morning wake up and bedtime... Chewy is like clock work... within 5 minutes, every time, except with the time change.
He gets a milk bone when he comes in in the morning, and when he comes in at bedtime... So he has incentive to get his business done, and hurry in.
7am he wakes me up...
He likes to go outside at night, but he is woofing at the door, promptly at 11pm.
I think they tell time the same way everything else does.