If you renew in advance, you always get a full year beyond your old expiry date. For example, suppose your membership expired on 2014 June 1, and on May 15 you paid in advance for another year. The result would be that your membership would expire on 2015 June 1, one full year after your original expiry date.
If it didn't work this way, people would be reluctant to renew in advance.
If you renew late, you still get a full year! For example, if your membership expired on July 1, 2014, and you didn't renew till September 1, your new expiry date would be September 1, 2015. Note that this is a more generous policy than many club memberships, where when you renew late, your new subscription is not for a full year from date of payment.