When you subtract date values directly, it gives you the difference in "days" provided date formatting is taken care. You will have to multiple the answer by 24*60*60 to convert it to seconds. See the below sql snippet which gives you a difference of 1 sec. You have use simpleDateFormat to convert the date to a acceptable format, subtract and multiple as needed. Hope this helps.
select (to_date('17-01-2013 09:13:46','DD-MM-RRRR HH24:MI:SS')- to_date('17-01-2013 09:13:45','DD-MM-RRRR HH24:MI:SS'))*(24*60*60) dt from dual