If you are checking for null as in above sql, you can use NVL instead of decode function. and your query might be something like following.

SELECT t.Start_Dat, t.End_Dat
FROM Tariff t
WHERE t.Start_Dat >=
(NVL((SELECT max(t.start_dat) FROM Tariff t WHERE t.Start_dat <= to_date('19/11/2003', 'dd/mm/yyyy')),
to_date('10/11/2003', 'dd/mm/yyyy')))


HTH.