Oracle 18c

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ALTER SESSION SET NLS_TIMESTAMP_TZ_FORMAT = 'YYYY-MM-DD HH24:MI:SS.FF9 TZH:TZM';
SELECT SYSTIMESTAMP AT TIME ZONE 'UTC',
       SYSTIMESTAMP AT TIME ZONE 'America/Denver'
FROM   DUAL;

SYSTIMESTAMPATTIMEZONE'UTC'
SYSTIMESTAMPATTIMEZONE'AMERICA/DENVER'

2022-06-14 18:38:24.479190000 +00:00
2022-06-14 12:38:24.479190000 -06:00

ALTER SESSION SET TIME_ZONE = 'America/Denver';
SELECT SYSTIMESTAMP, CURRENT_TIMESTAMP FROM DUAL;

SYSTIMESTAMP
CURRENT_TIMESTAMP

2022-06-14 19:38:24.486196000 +01:00
2022-06-14 12:38:24.486199000 -06:00