JSFiddle - React, Tailwind, and code Playground

HTML

<SELECT class="QTY">
<OPTION VALUE="1">1</OPTION>
<OPTION VALUE="2">2</OPTION>
<OPTION VALUE="3">3</OPTION>
<OPTION VALUE="38">38</OPTION>
</SELECT>

 <INPUT TYPE=SUBMIT id='Button' VALUE="Go">   
     <input id='checkin' />
     <input id='checkout' />
     <input id='duration' />

JavaScript

$('#Button').click(function(){
    var checkin = new Date();
    var duration = $('.QTY option:selected').val();
    var checkout = new Date(checkin);
    checkout.setDate(checkout.getDate() + Number(duration));
    
	$('#checkin').val(checkin.toLocaleDateString("ko-KR"));
    $('#checkout').val(checkout.toLocaleDateString("ko-KR"));
    $('#duration').val(duration);
});