a trigger for time

Hello to everyone,
I've got a set of tables where I need to store both the inserting and updating
time of a tuple. These columns are of coure two "time" columns. I'd like to
build a trigger (before) so that the time can be set up rightly when my
programs perform an insert or an update, but I cannot understand how to do
it. Does anybody know where I can find some example (or better is there
anybody who can send me an example)?

Thanks,
Luca

--
Luca Ferrari,
fluca1978(at)virgilio(dot)it