sum of a time column

Dear all,

I'm sorry if this sounds stupid or have been talked about before.

Suppose I have a field in my table that's called duration with type
'time without timezone'. How do I do sum on this field based on another
field let say called dateofevent?

SQL: SELECT SUM(duration) FROM durtable GROUP BY dateofevent;

gives me:

ERROR: Unable to select an aggregate function sum(time without time
zone)

Is this because of me still using PostgreSQL version 7.2.1 from Debian
Woody? Or some other stupidity?

TIA, Regards.
-arief