public function modified($time = null)
{
if ($time !== null) {
$date = $this->_getUTCDate($time);
$this->_headers['Last-Modified'] = $date->format('D, j M Y H:i:s') . ' GMT';
}
if (isset($this->_headers['Last-Modified'])) {
return $this->_headers['Last-Modified'];
}
return null;
}