Sferna4
The validity of the half fare credit relates to the purchase date of tickets (= date when the remaining credit is still available). The travel date on the ticket is absolutely irrelevant, the ticket inspectors do not care about at all how you paid for your tickets. If using a credit card, your ticket for next week also remains valid if the credit card expires tomorrow.
Therefore now you still may purchase e.g. saver day passes valid 5-6 months in the future, if you like.