In short, all financial transaction cards should show the card's expiration date in one of the following two formats: “MM / YY” or “MM-YY” — with the first being the by far most common for credit cards. The term "MM" means the two-digit of the month, while "YY" means the year of expiration. The abbreviation MM/YYYY on a credit or debit card refers to the two-digit month and four-digit year of the card's expiration date. The abbreviation MM / YY corresponds to the expiration date of the credit or debit card you are going to pay with. While testing, use only the credit card numbers listed here.