This is a factual issue. If there was no warning or description of the expiration date, they could not enforce it at all. If on the other hand it was evident at the point and time of sale, before you printed the ticket (e.g. a notice came up on the ticketing machine prior to the transaction being finalized), there would be no doubt but that they could enforce this limitation. What you describe falls in the middle: the information is apparently available, but not at the point of sale. Depending on surrounding circumstances, you may or may not be able to force a refund, etc. You'd have a good case if you had checked the website shortly before buying, and the new information was not yet up; you'd have less of a good case if you never checked or inquired into this. You also have to decide, if they don't want to refund, etc. you, whether it is worth the time and/or cost of fighting.