title: Why is ngModel non-assignable
body: <p>I have a date and i want it to have a filter on it so it would have the format i want for it.</p>
  
  <p>html:</p>
  
  <pre><code>&lt;input type="text" id="{{ID}}" ng-style="{color: color}" class="tdate ng-pristine ng-untouched ng-valid" ng-model="time.date | DatePlaceHolder"&gt;
  </code></pre>
  
  <p>filter:</p>
  
  <pre><code>function DatePlaceHolder($filter) {
      return function (value) {
          return $filter('date')(value, "dd/MM/yyyy") || "--/--/----";
      }
  }
  </code></pre>
  
  <p>It works fine but I keep getting this error:</p>
  
  <pre><code>Error: [ngModel: nonassign] Expression 'time.date | DatePlaceHolder' is non-assignable. Element: &lt;input type="text" id="{{ID}}" ng-style="{color: color}" class="tdate ng-pristine ng-untouched ng-valid" ng-model="time.date | DatePlaceHolder"&gt;
  </code></pre>