code: private function formatTimezone()
      {
          if (!$this->usingTz) {
              return "";
          }
          if ($this->externalOffset === 0) {
              return "Z";
          }
          
          $ext    = $this->externalOffset;
          $format = (0 < $ext) ? "-" : "+";
          $offset = abs($ext);
          $hour   = intval($offset / 60);
          $minute = intval($offset % 60);
          $format .= str_pad($hour,   2, '0', STR_PAD_LEFT);
          $format .= ':';
          $format .= str_pad($minute, 2, '0', STR_PAD_LEFT);
          return $format;
      }
docs: タイムゾーンを書式化します.
  @return string