MS Access and Date Calculations

I have a membership database, loosely based on the example provided with Access. What I need to do is be able to enter a payment for dues, and from that payment, calculate the member expiration date. I have a payment form that allows me to enter the purpose of the payment, but I am not stuck trying to get the expiration date correct. Any assistance or direction you could offer would be greatly appreciated.
Thank you in advance for your help.