DBMS_SCHEDULER. 

Default time zone change.

DBMS_SCHEDULER or DBMS_JOB And DST / Timezones Explained (Doc ID 467722.1)

SQL> ALTER SESSION SET NLS_TIMESTAMP_TZ_FORMAT ='DD/MM/YYYY HH24:MI:SS TZR TZD';

Session altered.

SQL> select dbms_scheduler.stime from dual;

STIME

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06/10/2021 07:09:12 CST6CDT CDT

SELECT *  FROM   dba_scheduler_global_attribute  WHERE  attribute_name = 'DEFAULT_TIMEZONE';  

SQL> exec DBMS_SCHEDULER.SET_SCHEDULER_ATTRIBUTE('default_timezone','Asia/Colombo');