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I believe each theater and/or production has its own policy on post dating. You have to check with the box office, or if you got your tickets through another source, such as Telecharge, you can call them. Usually you can find out if they'll substitute your tickets on the same day of the performance you want to see and you have to get tickets on a weeknight or weekday matinee. Of course, you may not get tickets comparable to the ones you originally have, but at least they might allow you to still see the show.