how to add one second to the date/time stamp in oracle

how to add one second to the date/time stamp in oracle

SELECT to_char('24-AUG-2015 13:00:00'/(24*60*60),'DD-MON-YYYY HH24:MI:SS') from dual;
SELECT to_char(to_date('24-AUG-2015 23:59:59','DD-MON-YYYY HH24:MI:SS')+1 /(24*60*60),'DD-MON-YYYY HH24:MI:SS') from dual;