title: Add a duration to a date by DateInterval
body: <blockquote>
    <p><strong>Possible Duplicate:</strong><br>
    <a href="https://stackoverflow.com/questions/2369082/what-date-time-functions-to-use-for-php-5-2">What date/time functions to use for PHP 5.2</a>  </p>
  </blockquote>
  
  
  
  <p>I want to add a duration to a date, I set this code :</p>
  
  <pre><code>$date = new DateTime(Date('r'));
  $date = $date-&gt;format("d-m-Y H:i:s");                       
  $interval = new DateInterval('PT1H2M3S');        
  $date = $date-&gt;add($interval);        
  echo $date;
  </code></pre>
  
  <p>and I get this error : Class 'DateInterval' not found</p>
  
  <p>Why is this and how can I modify the date without DateInterval?</p>