Credit Card Expiration Date Format Not Accepted Error

I hired a developer to make a checkout for me and so I do not know what version I have. It is not listed in the file. I have had several customers report and I have seen it myself where the credit card expiration date is not accepted no matter what format is tried. This happened to me using my own credit card one day and not the next on the same credit card. I have developed the rest of the site myself but am not sure how to fix this random problem.