title: AngularJS datefilter with $watchGroup
body: <p>I needed some datestamp formatting for my Angular UI-Calendar, in the case 2 instances of calender. To catch the date change i was using watchGroup, since the two calendars need same type of formatting. In the end I could not do the same as with two separated $watch for each calender:</p>
  
  <pre><code>        $scope.$watchGroup(['dt_start', 'dt_end'], function(mydate) {
  
          newdate = $filter('date')(mydate, "dd/MM/yyyy");
          console.log(newdate);
      });
  </code></pre>
  
  <p>output: <em>[Fri Jan 02 2015 00:00:00 GMT+0100 (W. Europe Standard Time), ...]</em></p>
  
  <p>with separated $watch's:</p>
  
  <pre><code> $scope.$watch('dt_start', function(mydate) {
          newdate = $filter('date')(mydate, "dd/MM/yyyy");
          console.log(newdate);
      });
  
      $scope.$watch('dt_end', function(mydate) {
          newdate = $filter('date')(mydate, "dd/MM/yyyy");
          console.log(newdate);
      });
  </code></pre>
  
  <p>and I get a dd/MM/yyyy. Am I doing something wrong on the watchgroup expression?</p>