From what I've read, dogs can smell time through the decay of scent particles. They don't know that an hour has passed, but they know that something doesn't smell as strongly. They understand day/night, but don't understand the concept of time the way we do.
Regardless, the dogs I've known have known the exact time for breakfast and dinner better than any humans I've known, and without any clocks to look at
My dog is an odd ball weirdo... he will have a whole bowl of food and not eat it until you spoon feed it to him or drop a few pieces on the ground first. Or he will eat it all in the middle of the night when I'm asleep. I end up just leaving a full bowl for him all the time cause I never know when he's hungry. I've tried to schedule him for food and he just won't go along with it at all.
