hur hittar man skillnaden mellan tidsstämpel i oracle?

Subtrahera dem. Resultatet blir ett INTERVAL datatyp som representerar, i detta fall, 17,092 sekunder.

SQL> ed
Wrote file afiedt.buf

  1  select to_timestamp( '27-FEB-12 02.11.31.910000000 AM', 'DD-MON-RR HH.MI.SS.FF9 AM') -
  2         to_timestamp( '27-FEB-12 02.11.49.002000000 AM', 'DD-MON-RR HH.MI.SS.FF9 AM')
  3*   from dual
SQL> /

TO_TIMESTAMP('27-FEB-1202.11.31.910000000AM','DD-MON-RRHH.MI.SS.FF9AM')-TO_
---------------------------------------------------------------------------
-000000000 00:00:17.092000000