Yes, you should still get the +10, even if it straddles over 2 different calendar months. Your challenge, depending how you plan your ADRs/park days, is that September park hours will probably not be out before you start booking. However by my workings, if your check in date is the 22nd August, you get the first 10 days of ADRs and that takes you to 31st August - which in turn eliminates your problem. You only get to advance book the first 10 days anyway, and after that you have to phone one day at a time.
