Validation not working for date format YYYY-MM

gistfile1.html

<div class="form-group">
    <label for="startDate" class="col-xs-12 control-label">Start date</label>
    <div class="col-xs-12">
        <input id="startDate" required name="experience[startDate]" data-fv-notempty-message="date is required" 
               type="date" placeholder="YYYY-MM" class="form-control revalidate start-date input-sm single-date-picker-month" 
               data-fv-date="true" data-fv-date-separator="-" data-fv-date-format="YYYY-MM" data-fv-date-message="Invalid date"/>
    </div>
</div>