Help with query

Dear list members:

I am new to the list and to SQL. I am using PostgreSQL 6.4.2 on Linux and
want to do the following query:
Given a table "works" with fields "idworker" (int4) and "timeofwork"
(datetime), i want to have a query who has the number of hours worked by
each "worker" in the form:
idworker hoursworked

I tried with:
SELECT idworker, COUNT(DISTINCT int8(timeofwork)/3600) as hoursworked
FROM works
GROUP BY idworker

but pgsql does not accept the DISTINCT inside the count (I read in a book
that there are some RDBMS that do accept it).
Is there another way to acomplish this?
Do I need to create a user-defined aggregate? If so, how should I do it?

Thanks in advance,
Guido Urdaneta

PS: Sorry for my bad english