They do have expiry dates on them, usually to the end of the year you purchase them, or the end of the year after, or 12 months. Really depends when you buy them or sometimes, where you buy them or if they are part of a package.
As above though, the value you paid for tickets can always be applied to the cost of new tickets regardless of where, when or how you purchased them. So while the ability to enter the parks with tickets you have in hand is dependant on the expiry date, the value of those tickets never expire. This also applies to vouchers which you need to exchange for actual tickets at the ticket booths outside the parks.
The only tickets I couldn’t exchange were the old ticket books my aunt gave me in the 70’s or 80’s which had individual tickets for each type of ride (e ticket, a ticket etc) before the parks changed to all inclusive tickets. I may have been able to when I fronted up with them in 1991 but it seemed to confuse the poor cast member so not wanting to be one of those Disney guests, I said it’s not an issue and just purchased a day ticket for $27-50 iirc.