trigger question

In an effort to monitor dates entered into the database I wrote a trigger and procedure to check and correct bad dates. I am encountering error -747 (Table or column matches object referenced in triggering statement). It did not make sense to me that you can not modify the same field that the user is trying to insert or update in a trigger. Can anyone please let me know if there is a way around this problem. thanks in advance. example: create trigger npsusers_upd_trupdate on nps_users referencing old as pre new as post for each row (execute procedure check_correct(post.date_last_chg) into date_last_chg); *** procedure check_correct modifies the date passed to it