If you haven't already done so, call. I'm in my 180+10 period today, and I tried booking HDD during that "+10 days" period online via both the old dining site and the new dining site and the later dates were not showing up. I just called and booked Xmas day without a problem. So there's something funky with the HDD and online reservation system. I had no problems reserving all my other dates via the old dining site.