Try this
<script type="text/javascript">
function getnoofdays(){
var oneDay = 24*60*60*1000; // hours*minutes*seconds*milliseconds
var firstDate = new Date(2008,01,12);
var secondDate = new Date(2008,01,22);
var diffDays = Math.round(Math.abs((firstDate.getTime() - secondDate.getTime())/(oneDay)));
Textbox3.Value=diffDays;
// alert(diffDays);
}
</script>
mark it answer if this is your solution
Thanks
ashish