Конфигурация

<link rel="stylesheet" href="//code.jquery.com/ui/1.12.1/themes/base/jquery-ui.css">
<script src="https://code.jquery.com/jquery-1.12.4.js"></script>
<script src="https://code.jquery.com/ui/1.12.1/jquery-ui.js"></script>
<script>
$( function() {
    $("#date_from").datepicker({ dateFormat: 'dd.mm.yy'});
    $("#date_for").datepicker({ dateFormat: 'dd.mm.yy'});
} );
</script>

<script type='text/javascript'>
function findRooms() {
    var dateParts = $('#date_from').val().split(".");
    var date_from = new Date(dateParts[2], (dateParts[1] - 1), dateParts[0]);
    dateParts = $('#date_for').val().split(".");
    var date_to = new Date(dateParts[2], (dateParts[1] - 1), dateParts[0]);
    if ((date_from == null) || (date_to == null)) {
        alert('Некорректный формат даты');
        return;
    }

    var ds = date_from.getFullYear()+'-'+('0'+(date_from.getMonth()+1)).slice(-2)+'-'+('0'+date_from.getDate()).slice(-2);
    var de = date_to.getFullYear()+'-'+('0'+(date_to.getMonth()+1)).slice(-2)+'-'+('0'+date_to.getDate()).slice(-2);
    var guests = $('#guests').val();
    var children = $('#children').val();

    window.location = 'https://flagmanair.com/booking/?hotel_id=&date_from='+ds+'&date_for='+de+'&guests='+guests+'&children='+children;
}
</script>

<input type='text' id='date_from' placeholder='Дата заезда'>
<input type='text' id='date_for' placeholder='Дата выезда'>
<select id='guests' placeholder='взрослые'>
   <option value='1' >1</option>
   <option value='2' >2</option>
   <option value='3' >3</option>
   <option value='4' >4</option>
   <option value='5' >5</option>
</select>
<select id='children' placeholder='дети'>
   <option value='0' >-</option>
   <option value='1' >1</option>
   <option value='2' >2</option>
   <option value='3' >3</option>
   <option value='4' >4</option>
   <option value='5' >5</option>
</select>
<button type='button' onclick='findRooms()'>Найти свободные</button>