fix(datepicker): add intermediary valid date check

- Add intermediary check for whether the date is valid due to an IE quirk of turning a valid date into an invalid date when using the date constructor

Closes #5872
Fixes #5865

Author: wesleycho

src/datepicker/datepicker.js

@@ -169,8 +169,9 @@ angular.module('ui.bootstrap.datepicker', ['ui.bootstrap.dateparser', 'ui.bootst
       self.activeDate = new Date();
     }
 
-    this.activeDate = ngModelCtrl.$modelValue ?
-      dateParser.fromTimezone(new Date(ngModelCtrl.$modelValue), ngModelOptions.timezone) :
+    var date = ngModelCtrl.$modelValue ? new Date(ngModelCtrl.$modelValue) : new Date();
+    this.activeDate = !isNaN(date) ?
+      dateParser.fromTimezone(date, ngModelOptions.timezone) :
       dateParser.fromTimezone(new Date(), ngModelOptions.timezone);
 
     ngModelCtrl.$render = function() {