Date difference in seconds

Hi ,
         Please consider the following schema and the query given below and
let me know how to get the results I want.

CREATE TABLE  temp (
       id                   INTEGER  NOT NULL,
       start_time       TIMESTAMP,
       end_time        TIMESTAMP,
       PRIMARY KEY (id)
);

Now I want the difference between start time and end time in seconds.
        Select date_part('seconds',start_time - end_time) from temp;  gives
me only the difference between the seconds value in both timestamps. It
ignores all the days, hrs, minutes etc.

My problem is that I want the actual difference between the two timestamps
expressed in seconds.

Please advice me how to write the SQL Query.

With Regards,
Raghunath T
Senior Software Engg,
MindTree Consulting Private Ltd,
Santa Clara, CA, USA