Conversion Hours Incorrect When Revert To Actual Date

I can't seem to understand why the hour is incorrect. Below query "dte_computation_on_data" is the old function they use to convert date and insert it to the table. Problem is when I revert it to the actual date the hour
is incorrect.

CODE
SELECT -- THIS HERE IS MY TEST TO REVERT TIME AND DATE ON THE FORMULA OF WITH RESPECT TO THEIR FUNCTION
to_char(TO_DATE('19700101', 'YYYYMMDD')+(tb1.dte_computation_on_data/86400),'MM/DD/YYYY') || ' ' ||
to_char(to_date(mod  (tb1.dte_computation_on_data,86400) ,'sssss'),'hh24:mi:ss ') revert_test,
systimestamp,tb1.dte_computation_on_data
from
( SELECT -- THIS IS THE FORMULA OF THE OLD FUNCTION THEY USE TO CONVERT DATE TO NUMBER AND INSERTED ON THE ROW
    floor((CAST(SYS_EXTRACT_UTC(systimestamp) AS DATE) - TO_DATE('19700101', 'YYYYMMDD')) * 86400) dte_computation_on_data
  FROM dual)tb1;

results
---------------------------------------------------------------------------------------
REVERT_TEST             SYSTIMESTAMP                            DTE_COMPUTATION_ON_DATA
05/19/2011 03:46:18     5/19/2011 11:46:18.005171 AM +08:00     1305776778