There is an issue in the implementation of the "On Change Script Code" property for Short Date or Timestamp fields in the form modeler. If you add a simple javascript alert to that property, for instance:

alert("boo");

The alert is not fired if you click on the field or change the date after starting the process instance... alert is fired only by clicking on "x" button.

Same javascript works for other field types such as Short text or Select box. Please see steps to reproduce for further details.