TimeStamp for insert/update - Mailing list pgsql-sql

hi!

I don't know is it possible or not, to make
a filed in any table which contains the insertion/update time
I know
CREATE TABLE ( instime timestamp default now(),.... );
works well for inserts..
but can I do the same for update?

thanks
    redax

ps; or it's a 'rules' question?
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