Expiration Date?!

So I guess I'm totally late to this party but I just found out that car seats have expiration dates. I seriously did not know this until yesterday. I was googling how to fit 5 car seats in my van (4 for my kids and 1 for my neighbor we carpool with) and I came across a facinating website - http://www.car-seat.org/ that has a forum where people ask all kinds of questions - including "how to fit 5 car seats in Honda minivan." So I surfed the site a bit and found out that car seats expire after 6 years for most brands and should not be used after that point. Turns out one of our seats expired on 7/23. I don't use that seat often - it's a backup but still - it's expired! Who knew..well apparently a lot of people because all the moms I asked at gymnastics knew about car seat expirations.