jQuery UI's DatePicker is dropdown by default, so the example you found is a little out of date I think.
https://jsfiddle.net/gRoberts/z4j227h6/
$( "#from" ).datepicker({
defaultDate: "+1w",
changeMonth: true,
numberOfMonths: 3,
onClose: function( selectedDate ) {
$( "#to" ).datepicker( "option", "minDate", selectedDate );
}
});
$( "#to" ).datepicker({
defaultDate: "+1w",
changeMonth: true,
numberOfMonths: 3,
onClose: function( selectedDate ) {
$( "#from" ).datepicker( "option", "maxDate", selectedDate );
},
beforeShowDay: function (date) {
var date1 = $.datepicker.parseDate($.datepicker._defaults.dateFormat, $("#from").val());
var date2 = $.datepicker.parseDate($.datepicker._defaults.dateFormat, $("#to").val());
return [true, date1 && ((date.getTime() == date1.getTime()) || (date2 && date >= date1 && date <= date2)) ? "dp-highlight" : ""];
},
});
The above code/fiddle shows that with a few minor changes, you can replicate the functionality in a dropdown.
HTH.