Actually, I believe that the answer is no.
The regular multi-day tickets expire seven days after the day of the first use (first day plus the next six consecutive days). The various annual passes expire after 365 days.
I think that the previous poster were indicating that you can buy tickets anytime and that they're good to use whenever you go in the future. But once you start using a multi-day ticket it follows the rules that were in place when you purchased it (like those that I outlined above).
Dick Taylor