Date_Part with time fields

Hi there,
I am trying to get Date_Part to work with time fields.  As it seems this
is not working:
 select date_part('hr',t.from) from times t;
ERROR:  No such function 'time_timespan' with the specified attributes

Any comment?

I am using [PostgreSQL 6.5.1 on i686-pc-linux-gnu, compiled by gcc
egcs-2.91.66]

Regards
Tim Mueller-Seydlitz