问如何在javascript中正确添加天数

EN

$("#btn").on('click', () => {
  let endday = new Date($("#207").data("day"));  
  let upperlimit = new Date(endday.setDate(endday.getDate()+120));
  // Have to add +1 to .getMonth() because JS handles months as 0-11 not 1-12
  let resultHtml = `${upperlimit.getFullYear()}/${upperlimit.getMonth() + 1}/${upperlimit.getDate()}`
  $("#result").html(resultHtml);
})

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<p id="207" data-class-index="0" data-day="2018/5/21">2018/5/21</p>
<button id="btn">Click to add 120 days</button>
<p id="result"></p>

const endDay = new Date('Thu Sep 13 2018 00:00:00 GMT+0900');

console.log("endDay", endDay);

var upperLimit = new Date(endDay);
upperLimit.setDate(endDay.getDate() + 120);

console.log("upperLimit", upperLimit);

Date.prototype.addDays = function(days) {
    var date = new Date(this.valueOf());
    date.setDate(date.getDate() + days);
    return date;
}

var date = new Date();

console.log(date.addDays(2));