Seventeen magazine suscription help?

So i suscribed to seventeen magazine on the july 6 and I chose bill me later. Anyways today I decieded to pay online with my grandmas credit card(she let me). I went to Customer Service and signed in with my email and zipcode. And for the pay my bill thing i filled out the credit card info and there was Expiration Date. What is that? I dont know how to fill it out?If it helps my suscirpition is 30 issues