public static function isoDate($timestamp = null)
    {
        $tz = date_default_timezone_get();
        date_default_timezone_set('UTC');

        if (is_null($timestamp)) {
            $timestamp = time();
        }

        $returnValue = str_replace(
            '+00:00', '.0000000Z', date('c', $timestamp)
        );
        date_default_timezone_set($tz);

        return $returnValue;
    }