datetimepicker arrow of moth with icon in background bootstrap-datetimepicker jsp javascript

i have an error with datetimepicker, one of the arrow of the month selecter, it have a icon in the background

if open the icon show , if i stay over the arrow change but no full in the space like the another side

i'm using struts jsp but i can't find the icon in the back. maybe overwrite it. Thanks

<script type="text/javascript" src="<html:rewrite forward="bootstrap"/>js/bootstrap-datetimepicker.min.js"></script>

$('#datetimepickerInit').datetimepicker({format: 'DD/MM/YYYY', locale: 'de'});

<div class="input-group input-group-sm" id="datetimepickerInit">
   <html:text property="filterInitTaskDate" styleId="filterInitTaskDate" styleClass="form-control input-sm"></html:text>
    <span class="input-group-addon"><span class="glyphicon glyphicon-calendar"></span></span>
    </div>