The nice people over at Disney Dining will book up to what they think is the latest time if you want to book for dinner. They will tell you to call back to update it if you were looking for something later. Other than that, just make your PS's. Don't get frustrated if this is the case.
For example, I made a PS at Sci Fi (90 days before) for 5:55pm for dinner on 11/3 and the rep told me that that's the latest the system was allowing her to book because the hours for Nov. haven't come out yet. We want it at 6:55pm instead and as soon as it comes out I'll call to update.