JSFiddle - React, Tailwind, and code Playground

HTML

<form>
    start date(yy/mm/dd):<br>
    <input type="text" id="start_date" name="firstname" value="2015/1/1" >
<br>
    end date(yy/mm/dd):<br>
    <input type="text" id="end_date" name="lastname" value='2015/5/22'>
    <input type='button' id='get_days' value="get days"></input>
</form>
<div class='days'>ee<div>

JavaScript

Date.daysBetween = function( date1, date2 ) {
  //Get 1 day in milliseconds
  var one_day=1000*60*60*24;

  // Convert both dates to milliseconds
  var date1_ms = date1.getTime();
  var date2_ms = date2.getTime();

  // Calculate the difference in milliseconds
  var difference_ms = date2_ms - date1_ms;

  // Convert back to days and return
  return Math.round(difference_ms/one_day); 
}
$('#get_days').click(function(){
d1 = new Date($("#start_date").val());
console.log(d1);
d2 = new Date($("#end_date").val());


$('.days').text(Date.daysBetween(d1, d2)); 
    
});