How to substract timezones in Oracle

select to_char(systimestamp at time zone 'GMT','HH24') -  TO_CHAR(systimestamp,'HH24')  from dual


Substract Hours from Timestamp:

select (to_date('2016/07/01 03:10:04 PM', 'yyyy/MM/dd HH:MI:SS AM') -  interval '5' hour ) from dual