title: Adding months to DateTime with DateInterval changes original date to match new date
body: <p>I have this pretty simple code:</p>
  
  <pre><code>$start_date = new DateTime($post['start_date']);
  $end_date = $start_date-&gt;add(new DateInterval('P6M'));
  echo $start_date-&gt;getTimestamp(); // 1351836000
  echo $end_date-&gt;getTimestamp(); // 1351836000
  </code></pre>
  
  <p>Of course, both end up as the same timestamp because adding the date interval affects the original $start_date. So how do I go about this so I can keep the original $start_date yet add 6 months to it in another variable?</p>
  
  <p>I tried this with no luck:</p>
  
  <pre><code>$start_date = new DateTime($post['start_date']);
  $start_date_actual = $start_date;
  $end_date = $start_date_actual-&gt;add(new DateInterval('P6M'))-&gt;getTimestamp();
  </code></pre>