public static function getISO8601($timestamp=false) {
        if ( $timestamp === false ) {
            $dt = new \DateTime();
        } else {
            $format = 'Y-m-d H:i:s';
            $dt = \DateTime::createFromFormat($format, $timestamp);
        }

        // 2017-08-16T16:26:31-1000
        $iso8601 = $dt->format(\DateTime::ISO8601);

        // 2017-08-20T10:34:05.000Z
        $iso8601 = str_replace('-1000','.000Z',$iso8601);

        return $iso8601;
    }