How to find last day of month in jquery datetime picker?

function getLastDayOfYearAndMonth(year, month){
    return(new Date((new Date(year, month + 1, 1)) - 1)).getDate();
}

jQuery Date picker ui select date range picker

Jquery UI Datepicker select date range between Two days

$( "#start_date" ).datepicker({
   minDate: new Datenew Date(1999, 10 - 1, 25),
   maxDate: new Datenew Date(1999, 10 - 1, 30),
   defaultDate: "+1w",
   changeMonth: true,
   numberOfMonths: 2,
   onClose: function( selectedDate ) {
      $( "#end_date" ).datepicker( "option", "minDate", selectedDate );
   }
});

$( "#end_date" ).datepicker({
   minDate: new Datenew Date(1999, 10 - 1, 25),
   maxDate: new Datenew Date(1999, 10 - 1, 30),
   defaultDate: "+1w",
   changeMonth: true,
   numberOfMonths: 2,
   onClose: function( selectedDate ) {
      $( "#start_date" ).datepicker( "option", "maxDate", selectedDate );
   }
});