private function getMaxMTime($theDirName)
  {
    $iterator = new RecursiveIteratorIterator(
      new RecursiveDirectoryIterator($theDirName, FilesystemIterator::SKIP_DOTS),
      RecursiveIteratorIterator::SELF_FIRST);

    $mtime = null;
    foreach ($iterator as $file_info)
    {
      $mtime = max($mtime, $file_info->getMTime());
    }

    return $mtime;
  }