We didn't so much teach this trick, as he learned it on his own, but we got an automatic feeder for him to help us with his diet. It feeds him 6x per day, and since we're no longer the ones giving him food, he has stopped begging us for food at all hours.

In any case, a few months ago, we noticed him in the dining room, staring at the grandfather clock. Then the clock chimed, and he ran over to the feeder and stared at it until a few seconds later, when it dispensed the food. He doesn't do this, except around feeding time.

So, it's just pavlovian: he's learned to associate the clock chime with the feeder, and has a general idea when feeding time is based on how hungry he is. Still, it's a neat party trick to tell guests we have a cat who can read a clock and tell time.