Adding seconds to a in PostgreSQL

The trick is to create a fixed interval and multiply it with the number of seconds in the column:

SELECT start_time, 
       expiration_time_seconds, 
       start_time + expiration_time_seconds * interval '1 second'
FROM whatever 
ORDER BY start_time;

        start_time          | expiration_time_seconds |          end_time
----------------------------|-------------------------|----------------------------
 2014-08-05 08:23:32.428452 |                  172800 | 2014-08-07 08:23:32.428452
 2014-08-10 09:49:51.082456 |                    3600 | 2014-08-10 10:49:51.082456
 2014-08-13 13:03:56.980073 |                    3600 | 2014-08-13 14:03:56.980073
 2014-08-21 06:31:38.596451 |                    3600 | 2014-08-21 07:31:38.596451