How to use the TIMESTAMP_TO_SCN function

select timestamp_to_scn(to_timestamp('2020-05-11 14:36:22', 'YYYY-MM-DD HH24:MI:SS')) "scn" from dual;

scn
----------
20325895

Reversed:

select scn_to_timestamp(20325895) "timestamp" from dual;

timestamp
----------------------------------
11-MAY-20 02.36.20.000000000 PM