title: moment.js add/subtract days without affecting the original date
body: <p>How do you add or subtract days to a default date using moment.js?</p>
  
  <p>I am trying to get the start and end dates of the week like below:</p>
  
  <pre><code>const current = moment.tz('2016-03-04', 'America/Los_Angeles');
  const startOfWeek = current.startOf('isoWeek').weekday(0);
  const endOfWeek = current.endOf('isoWeek').weekday(6);
  </code></pre>
  
  <p>When calling <code>endOfWeek</code>, I am getting the expected value. 
  However, my problem is that <code>startOfWeek</code> is overridden by the <code>endOfWeek</code> value.</p>
  
  <p>I wanted to get the value of both <code>startOfWeek</code> and <code>endOfWeek</code> </p>