Sorry, I don't have a fix, but I believe I have some info that may help.
I am experiencing the same error message, "this.getDateField() is null" and have used Firebug to determine my issue is related to the date field having a setting of No for "Allow typing in the field".
The setUp() function in date.js does a === comparison of this option, but the No setting comes across as "0" which would == false, but not === false. This causes code execution to proceed into the else statement where it shouldn't go.
I tested changing the code and was able to make it past that section successfully, but am now to a new error "this.element.getElement("img.calendarbutton") is null" because the current user does not have Editable permission and the calendar doesn't exist.