Bootstrap Datepicker viewmode Only Month show

<div class="input-append date">

 <input  type="text" readonly="readonly" name="date"  id="datepicker">    
 
</div>      

$("#datepicker").datepicker( {
    format: "mm",
    viewMode: "months", 
    minViewMode: "months"
});

Bootstrap Datepicker viewmode  Only Month show