Solution:
This information is available in the jQueryUI documentation. Here’s an example of restricting the user to only allow a date 20 days in the past and 1 month + 20 days into the future. Replace your current $("#datepicker").datepicker();
declaration with the following
<script>
jQuery( function() {
$( "#datepicker" ).datepicker({ minDate: -20, maxDate: "+1M +10D" });
} );
</script>