adding time to a datetime field ... how?

I hate coming up with subjects...

I have a table with two fields: start_time::datetime and
acctsessiontime::int4...I want to add the second to the first, to give me
a stop_time...

if I do:

select start_time,start_time+acctsessiontime from log;

I get a stop_time 'acctsessiontime' days after start_time, instead of
'acctsessiontime' seconds later...

Is there some way of telling it to pass seconds instead of days?

Thanks

Marc G. Fournier                   ICQ#7615664               IRC Nick: Scrappy
Systems Administrator @ hub.org 
primary: scrappy@hub.org           secondary: scrappy@{freebsd|postgresql}.org