title: Date returns NaN after instantiation with a timestamp
body: <p>I saw <a href="https://stackoverflow.com/questions/6963311/add-days-to-a-date-object">this question</a> , so what is did was :</p>
  
  <pre><code>let date = 1507135378538;
  
  let date2 = new Date(date).getTime(); //NaN
  let dateIn30Days = date2 + (30 * 24 * 60 * 60 * 1000);//NaN
  </code></pre>
  
  <p>What I'm doing wrong?
  thanks</p>