When you renew your account, no matter whether you renew it exactly on its expiration date or a few days/weeks/months earlier, the renewal will count from the account's expiration date.
For example, if your account expires on July 4th 2009 and you renew it for one year on June 2nd 2009, the new expiration date will be July 4th 2010, not June 2nd 2010.