I believe they open at 180 days like everything else, but for whatever reason, the web site doesn't reflect that. You should call.
Another thing you can try (worked for me last year) is going to the dinner packages section of the dining web site, and then manually entering the date you want including a zero - ex: 07/14/12. For whatever reason, when you manually enter the date using that leading zero, it would pull up availability. I had to do this for my last trip on my 180th day in March 2011, and it worked for me then.