December 31 2012 "not a valid date"
trying to enter a due date of December 31, 2012 fails, either by entering text or by using the pull-down calendar.
Although I can confirm that "December 31, 2012" isn't accepted as a valid date, but I can't reproduce this issue when using the drop-down calendar.
While having the "MM-DD-YYYY" date format selected in your Settings, you can use the following options to set such date:
december 31 2012