Calendar.setup Calendar popup not display proper on Magento admin panel

HTML Code

<td class="label"><label for="tran_date">Date</label></td>
<td class="value">
    <input name="deposit[created_at]" id="tran_date" type="text" class="input-text" value="<?php echo Mage::getModel('core/date')->gmtDate('m/d/Y'); ?>"/>
    <img src="<?php echo $this->getSkinUrl('images/grid-cal.gif') ?>" class=""
        id="tran_date_trig"/>
    <div id="calendar--flat"></div>
</td>
</tr>

SCRIPT

 <script type="text/javascript">
// <![CDATA[
Calendar.setup({
flat: 'calendar--flat',
ifFormat: '%m/%e/%Y',
button: 'tran_date_trig',
align: 'Bl',
singleClick: true,
onSelect: function(dateText,selectedDate) {
    jQuery('#tran_date').val(selectedDate);
    jQuery('#calendar--flat').hide();
}
});
// ]]>
</script>

CSS

#calendar--flat {
    position: absolute;
}