You can book a reservation up to 500 days from the day you want to check OUT.
Normally, their figures deal with checking IN, but for booking, the window is 500 days from the date of checkout.
Because park ticket expiration doesn't begin until you first USE the ticket, you can buy tickets now. Don't tell them the tickets are for a 2007 trip. You can buy now at the 2006 prices.
HTH