I saw that it said those dates for validity, however I made an order yesterday and already get the gc by email yesterday as well and all it said was that it expires 12/24, did not say anything about the start date. I tested if I can use it towards payment on an order right away and I was able to apply it (although I did not submit the order but I do believe it would go through).

In order to extend the validity of the gc, you can buy something for $75 and then return it and you will get a merchandise credit which operates the same way as a gc but has no expiration.