title: AngularJs cannot see updated value of input and setting ng-invalid
body: <p>Datepicker gets date and puts it into <code>input</code> field. Input marked as <code>required</code> but AngularJS cannot see changes and mark it as valid. Why?</p>
  
  <p><strong>HTML</strong></p>
  
  <pre><code>&lt;div class="form-group" ng-class="{ 'has-error': myForm.inputDatetimeStart.$invalid }"&gt;
      &lt;label for="datetimepicker_start" class="col-sm-3 control-label"&gt;Start date and time: &lt;/label&gt;
      &lt;div class="col-sm-8"&gt;
          &lt;input class="form-control" name="inputDatetimeStart" ng-model="datetime_start" id="datetimepicker_start" type="text" required&gt;
      &lt;/div&gt;
  &lt;/div&gt;
  </code></pre>
  
  <p><strong>JS</strong></p>
  
  <pre><code> angular.module('myApp', ['ajoslin.promise-tracker', 'ngRoute'])
       .controller('myCtrl', function ($scope, $http, $log, promiseTracker, $timeout){
         jQuery('#datetimepicker_start').datetimepicker({   });
         ...
  
       }
  </code></pre>