title: Use DateTime::modify() to change time of day
body: <p>I want to change the time of day in a PHP <code>DateTime</code> object like this:</p>
  
  <pre><code>$date = new DateTime('2015-07-20 0:00');
  var_dump($date-&gt;format(DateTime::ISO8601));
  
  $date-&gt;modify('18:30:00');
  var_dump($date-&gt;format(DateTime::ISO8601));
  </code></pre>
  
  <h3>Expected output</h3>
  
  <pre><code>string(24) "2015-07-20T00:00:00+0200"
  string(24) "2015-07-20T18:30:00+0200"
  </code></pre>
  
  <h3>Actual output</h3>
  
  <pre><code>string(24) "2015-07-20T00:00:00+0200"
  string(24) "2015-07-20T00:00:00+0200"
  </code></pre>
  
  <p>How can I achieve this?</p>