Oracle 21c

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alter session set nls_date_format = 'YYYY-MM-DD HH24:MI:SS'
alter session set nls_timestamp_format = 'YYYY-MM-DD HH24:MI:SS.FF1'
alter session set nls_timestamp_tz_format = 'YYYY-MM-DD HH24:MI:SS.FF1 TZR'
select SYSDATE, SYSTIMESTAMP DBTIMEZONE from dual;

SYSDATE
DBTIMEZONE

2022-09-06 18:06:04
2022-09-06 18:06:04.9 +01:00

select SYSTIMESTAMP, SYSTIMESTAMP at time zone 'UTC' from dual;

SYSTIMESTAMP
SYSTIMESTAMPATTIMEZONE'UTC'

2022-09-06 18:06:04.9 +01:00
2022-09-06 17:06:04.9 UTC

select cast(SYSTIMESTAMP at time zone 'UTC' as date) from dual;

CAST(SYSTIMESTAMPATTIMEZONE'UTC'ASDATE)

2022-09-06 17:06:04

-- original
select round((SYSDATE - date '1970-01-01')*24*60*60) from dual;

ROUND((SYSDATE-DATE'1970-01-01')*24*60*60)

1662487564

select round((cast(SYSTIMESTAMP at time zone 'UTC' as date) - date '1970-01-01')*24*60*60) from dual;

ROUND((CAST(SYSTIMESTAMPATTIMEZONE'UTC'ASDATE)-DATE'1970-01-01')*24*60*60)

1662483964