select sum() possible or not ?

Hello,

I'm using PostgreSQL 7.1

I have a table like :

date logon     logoff
---------------------------------
01-01-02 | 12:02:03 | 12:04:23
01-01-02 | 12:05:13 | 12:08:13
01-01-02 | 13:02:03 | 13:04:23
01-01-02 | 14:02:03 | 14:04:23
01-02-02 | 08:59:03 | 09:09:23
........

and I want to do the following select :

select sum(logoff - logon) from online;

but I receive a error that it's not possible to select a aggregate function
sum(time).

Does someone have an idea why, is there a configuration switch which could solve
the problem?

Thanks

Jörg Naumann