The resort confirmation check-in only allows up to 180 days. If you put in that as your check-in date, it will open up the extra days. (No, it shouldn't have any effect on your actual resort reservation).
---
On another note, are those making either 180 or 190 day reservations getting the wording about the dining plan? Something along the lines of "You may pay for this meal with your
Disney Dining Plan" when you book? Just digging a little deeper into how the interaction between resort and dining.
EDIT:
The calendar "pop-up" is not your traditional old-style pop-up window that the blockers catch. It's a piece of Javascript (a subset of Java, which focuses on scripting to produce executable ("runnable") pieces of a web application) that displays a piece of a page within the current page. Different from the traditional popups that display a different page in a while new (albeit controlled) window. There's quite a bit more technical stuff behind it, but I'll spare you some

.
And... hmm. In looking into the page (figures, I saw this in source view before normal view...)
The wording of "more than 180 days before their stay" suggests that I very well may be wrong in my initial assumption (though, since the page accepts both false dates and false confirmations, it's still broken
). The plot thickens
.