Adding time

Hi,
I have a table with process_id and process_started_at and
process_completed_at which are both datetime datatype.
I need to calculate the total time taken for a particular process for a
given date-range.
and calculate average time per day per process_id.
How can I do it I have MS SQL 2003.
If I add the 2 datetime variables it increments the day by 1 after
every 24 hrs but that is not what I want.
Example if I have
date1 = '2005-01-01 20:20:30'
date2 = '2005-01-01 20:25:20'
then I want the result as '40:45:50' I am not concerned with the date
part.
I tried creating a function which would add the two times but then how
do I get the average? Do I have to write another function which will
convert the total time into seconds and then devide by total number of
days and then convert back to hr:min:sec or is there is easier way to
do it?
Does MS SQL have any simple methode which will convert the time to
seconds and seconds to hr:min:sec?
Thanks for your time and expertise
Ashoo> Does MS SQL have any simple methode which will convert the time to
> seconds and seconds to hr:min:sec?
For the latter, see http://www.aspfaq.com/2271