SELECT * FROM   emp AS OF SCN TIMESTAMP_TO_SCN(SYSTIMESTAMP - 1/24);

SELECT * FROM  emp AS OF TIMESTAMP SCN_TO_TIMESTAMP(20);

DECLARE
  l_scn        NUMBER;
  l_timestamp  TIMESTAMP;
BEGIN
  l_scn       := TIMESTAMP_TO_SCN(SYSTIMESTAMP - 1/24);
  l_timestamp := SCN_TO_TIMESTAMP(l_scn);
END;
/