Oracle 11g Release 2

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alter session set NLS_DATE_FORMAT = 'dd-MON-yy hh24:mi:ss';
SELECT 
    systimestamp,
    190903210000 num,
    TO_TIMESTAMP(TO_CHAR(190903210000), 'YYMMDDhh24miss') num_as_timestamp,
    (CAST(systimestamp AS DATE) - CAST(TO_TIMESTAMP(TO_CHAR(190903210000), 'YYMMDDhh24miss') AS DATE)) * 60 * 60 * 24 diff
FROM DUAL;


SYSTIMESTAMP
NUM
NUM_AS_TIMESTAMP
DIFF


08-JAN-21 11.13.52.463906 PM +00:00
190903210000
03-SEP-19 09.00.00.000000000 PM
42603232.00000000000000000000000000000008


SELECT 
    systimestamp,
    190903210000 num,
    TO_DATE(TO_CHAR(190903210000), 'YYMMDDhh24miss') num_as_date,
    (CAST(systimestamp AS DATE) - TO_DATE(TO_CHAR(190903210000), 'YYMMDDhh24miss')) * 60 * 60 * 24 diff
FROM DUAL;


SYSTIMESTAMP
NUM
NUM_AS_DATE
DIFF


08-JAN-21 11.13.52.465452 PM +00:00
190903210000
03-SEP-19 21:00:00
42603232.00000000000000000000000000000008