So I found the easiest way to do this was to create a calendar control with a default value of your date.  You can then create a rule that hides this control (I called this control Test Date (if {Self}!="" hide control)). 

Then create your rule on the date/time control that you want your people to fill out.  Your rule would be Test Date<{Self} set to invalidate and add whatever text you'd like. 

Testing this in my environment worked.