-- Oracle 21c: JSON_SCALAR Function 

SELECT 
  JSON_VALUE(JSON_SCALAR(current_timestamp), '$.timestamp()')
FROM
 DUAL
;

SELECT 
  JSON_SCALAR(to_timestamp_tz('2019-05-03 20:00:00 -8:30',
                              'YYYY-MM-DD HH24:MI:SS TZH:TZM'))
FROM
 DUAL
;