Allow date format for the input binding.

<div class="dropdown">
  <a class="dropdown-toggle" id="taskStartToggle" role="button" data-toggle="dropdown" data-target="#" href="#">
    <div class="input-group">
      <input placeholder="yyyy-MM-dd" type="text" class="form-control" data-ng-model="task.start" />
      <span class="input-group-addon"><i class="glyphicon glyphicon-calendar"></i></span>
    </div>
  </a>
  <ul class="dropdown-menu" role="menu" aria-labelledby="dLabel">
    <datetimepicker data-ng-model="task.start" data-datetimepicker-config="{ format: 'yyyy-MM-dd', minView: 'day', dropdownSelector: '#taskStartToggle' }"/>
  </ul>
</div>