MySQL-esque sec_to_time() function

Hello,

I'm converting from MySQL to PostgreSQL (actually flipping/flopping back) and have a question:

MySQL has a cool function sec_to_time() which converts your number of seconds to hh:mm:ss

I've read thru the mailing lists and am basically trying to implement the following:

MySQL:
select sec_to_time(sum(unix_timestamp(enddate) - unix_timestamp(startdate))) from foo;

PostgreSQL:
select XXX(sum(date_part('epoch',enddate) - date_part('epoch',startdate))) from foo;

I just need to know what XXX is/can be. I've tried a lot of combinations of the documented functions and come up with useless conversions.

Thanks,
George Johnson
gjohnson(at)jdsc(dot)com

PS: i can't find documentation on how to load the functions in contrib. I can compile and
install them O.K., but not sure how to make them load.