Validate the date object using the modelValue

src/date.js

@@ -56,7 +56,7 @@ angular.module('ui.date', [])
 
           // Update the date picker when the model changes
           controller.$render = function () {
-            var date = controller.$viewValue;
+            var date = controller.$modelValue;
             if ( angular.isDefined(date) && date !== null && !angular.isDate(date) ) {
               throw new Error('ng-Model value must be a Date object - currently it is a ' + typeof date + ' - use ui-date-format to convert it from a string');
             }