title: Fatal error: Call to undefined method DateTime::add()
body: <pre><code>&lt;?php
  function OtherDAY($start,$total){
  
      $date = new DateTime("$start");
      $date-&gt;add(new DateInterval('P'.$total.'D'));
      $finish=$date-&gt;format('Y-m-d');
      return $finish;
  
  }
  
  $start=date("Y-m-d");
  $otherday=OtherDAY("$start",'15');
  die($otherday);`
  ?&gt;`
  </code></pre>
  
  <p>It is showing me "not a date format" after i kill it, <strong>how to fix this</strong>?</p>