While it'll be second grade before most kids can learn to read a clock, younger ones can begin to grasp the concepts of "later" and "soon." How you can help:
Provide a reference point. When you tell your child that supper will be ready in five minutes, explain, "That's about how long it'll take you to listen to your new Wiggles song.
Read all about it. Almost any picture book can be used to talk about time. As you read together, ask him, "Is it morning or night at the bunny's house?" or "How long do you think Goldilocks slept?" Your child's practice with these kinds of time words will help him understand when you have to say, "No, tomorrow is the day you have a playdate."
Show and tell. Have him tear off a calendar page each evening to mark the passage of time. Let him set the timer for cookies, or give him a stopwatch so he can see how many blocks you can walk together in ten minutes. And don't be surprised if he starts saying "Five minutes!" when you tell him it's time to put away his toys.