As long as Expiry Date is a DateTime object, you should have relatively little trouble. Now, I'm assuming here that you are applying this at the client side, seeing that you haven't included SQL Server or other database in your tags, I'm not going to cover doing this as a query and I'm assuming that you have retrieved this data already. Suppose you mapped Expiry Date into a C# property called ExpiryDate, then all you need to do is:
bool shouldRenew = ExpiryDate.AddMonths(-1) > DateTime.Now;