Joe Offered to Sell Harry His Car and Agreed to Hold

Joe offered to sell Harry his car and agreed to hold the offer open for one week, expiring Saturday at noon. The Sunday after the offer was said to have expired, Harry phoned Joe to tell him that he decided to accept his offer and purchase his car. Joe said at that time that he had decided to keep the car and was just about to phone Harry to revoke the offer. Harry said, "Too bad," and demanded performance of the contract. Explain the legal responsibilities of the parties.