public function format($format, $tz_format = null)
    {
        if (is_int($format)) {
            $format = self::getFormatById($format);
        }

        $out = parent::format($format);

        if ($tz_format !== null) {
            $out .= ' ' . $this->formatTZ($tz_format);
        }

        return $out;
    }