I wouldn't worry about a shampoo's expiration date much. (depending on circumstances, but assuming that it is one you've bought and just not used too quickly.... if you found a bottle in the back of a random cupboard in an old barn/shed somewhere OTOH, toss it!).
In general shampoo/conditioner has a 1 year lifetime from the time it's been opened. After that it starts to breakdown, I have no idea if it's still good or not though, and if it might or might not cause an allergic reaction. Imo i'd just toss it in the trash lol.