Hm, I was not able to reproduce this issue. Make sure you're entering the expiration date/time in the proper format: YYYY-MM-DD HH:MM:SS (e.g., 2010-12-31 23:59:59).
Also, bear in mind that the Max. Use Count determines how many times the discount can be used ever, not how many times each buyer can use it. If you'd set Max. Use Count = 1, then the discount would become expired after the first time anyone used it. Since all cart orders are anonymous before checkout, there's no way to track how many times each individual buyer uses a common discount; therefore, if you want buyers to be able to apply a given discount only once, you'd need to set up your discount with a list of Discount Codes and Max. Use Count = 1, then issue each buyer their own, personal Discount Code to use from that list.