When I bought my infant car seat brand new, I knew nothing about expiration dates on them. When I found out about them I looked at ours and it was a year and a half old when I bought it brand new. Had my youngest not gotten over 20 pounds by 6 months, we would have had to have the debate of using it even though it was expired or buying a new one for a few months.
He’s only 4 years younger than his older sister, but car seats are only good for 6 years and it had a year and a half in sitting in the box. Moral of the story: If possible ask to see the expiration date on the one you are getting if you want it to be able to use it for a long time.