You can actually renew after your expiration date and still get the $20 off. However no matter when you renew it will always be good for a 365 days from your original expiration date. So if your pass expires September 1, 2006 and you renew (either before then or within a few days after) it will expire on September 1, 2007. Now if you renew in 2007, then it will expire August 31, 2008, since 2008 is a leap year. So every 4 years your expiration date would change by 1 day.