title: How to add days correctly in javascript
body: <p>I tried to add days to somedays by clicking calendar like tables.</p>
  
  <p>first, I can get endday by clicking,and then,I would like to add days 120 to it.</p>
  
  <pre><code>&lt;td id="207" data-class-index="0" data-day="2018/5/21"&gt;21&lt;/td&gt;
  </code></pre>
  
  <p>my code is like below</p>
  
  <pre><code>endday=new Date($(this).data('day'))
  
  console.log("enddate",endday);        
  
  upperlimit=endday.setDate(endday.getDate()+120);
  
  console.log("upperlimit",upperlimit);   
  </code></pre>
  
  <p>When I tried to add days, it seems return current time. like this</p>
  
  <pre><code>enddate Thu Sep 13 2018 00:00:00 GMT+0900 (Japan Standard Time)
  upperlimit Fri Dec 20 2019 11:49:34 GMT+0900 (Japan Standard Time)
  </code></pre>
  
  <p>I wonder why this issue incurred, if someone has some experience like this,please let me know how to fix.</p>
  
  <p>Thanks</p>