If the offer states that it is valid through the end of May, they will not let you use it in Sept.
But they may have other promotions going during that time, like free dining.
I got a pin for the $500 giftcard and it says valid through April 16th, but when I called they said it was valid through June 6th. So they do extend pin code offers, but it did have a new end date, so I couldn't book it for September.
Extending would be if the pin was offered through May and they changed it to september. If the email said May but the pin was actually through september the whole time, that is not extended. I saw that all the time. People would call in and say "Oh I wish this pin was good until X date" and I would check the pin's dates and it was actually good for the dates they wanted even though their email said otherwise. Just like last december's promo that said it wasnt good for 12/4-12/16 or something like that but people were able to get it for those dates.