Cosmetic expiration date

Do you use your cosmetics according to their expiration dates?

Actually, even if I just use it, it’s not really obvious whether it’s bad for my skin or not, so I just keep using it...

It probably isn't good for the skin, right?

Also, cosmetics usually say to use them within a few months of opening; do you follow that rule when using them?

I'm still using something that's really old...