The following SQL will subtract 15 minutes from an Oracle Timestamp, and 20 hours from a timestamp.
--15 Minutes Ago
select systimestamp,
cast(to_char(sysdate - (15/(24*60)),'dd-mon-yyyy hh:mi:ss') as timestamp)
from dual;
--Yesterday 20 hours ago
select systimestamp,
cast(to_char(sysdate - (1200/(24*60)),'dd-mon-yyyy hh:mi:ss') as timestamp)
from dual;