A discount card automatically expires one year after being issued. In order to be used again after this time, it must be renewed.
Upon scanning an expired discount card, it will offer you the opportunity to renew it. Simply say yes, and the price of the card will be added to the current invoice. Note that the card WILL apply to that transaction.
To view the expiration date of an unexpired card, go to the more information screen for the customer after selecting the customer on the invoice; it will be listed below their address.