Federation Internationale de Philatelie through Commission for Maximaphily says the cancellation must be carried out within the period of validity of the postage stamp and as close as possible to the date of issue.
I do not create these cards for competitive exhibitions, I do create them on the bases of my own aesthetical tastes. The state that issued the stamp is the state where the subject is placed, and I look for all the issued stamps through the time with a particular subject never carring if the stamp's validity is off or not. Also, I prefer the postcards issued by the same state as the postage state. Concerning the cancellation, when I send a postcard like these by mail, and the stamp is not valid anymore, I cancel it to order (CTO) at the nearest post office to the subject, and I put some valid postage stamps on the other side of the postcard. So, for me it is more important the accordance of the place on the postal marking rather than the date.
As I said, I don't care about exhibitions, I create them for sending them by mail for private use.