how to limit the selection of dates .. Date range validation specifically 90 days

this is what I have done

<script type="text/javascript">
$(function () {
$("[id$=txtOnDate]").datepicker({
useMode: 3,
dateFormat: "d-M-yy",
firstDay: '1',
onSelect: function (dateStr) {
$('#txtAppDate').datepicker('option', 'defaultDate', dateStr);
$('#txtAppDate').datepicker('option', 'minDate', dateStr);
}
});
});
</script>
<script type="text/javascript">
$(function (){
$("[id$=txtAppDate]").datepicker({
useMode: 3,
dateFormat: "d-M-yy",
firstDay: '1',
maxDate: "+60D",
});
});
</script>


the above code takes only the current date or todays date into account. but I want it to take a specific date that the user selects

Thanks