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

Two different ways to retreive the current System Change Number

Use either of the following methods:

SQL> col current_scn format 9999999999999
SQL> select current_scn from v$database;

   CURRENT_SCN
--------------
 6525378448555

SQL> col GET_SYSTEM_CHANGE_NUMBER format 9999999999999
SQL> select dbms_flashback.get_system_change_number from dual;

GET_SYSTEM_CHANGE_NUMBER
------------------------
           6525378448442

Oracle officially recommends tht if your goal is to obtain an SCN from within an application, then you should use the dbms_flashback.get_system_change_number function.