Forum OpenACS Q&A: Porting triggers -- replace with ON UPDATE?

I'm new to Postgres, so forgive me if this is a really simple question. I've looked all around through the Postgres docs and through these Forums, as well as the two porting guides on this site.

If I'm porting a table from Oracle to Postgres that has a trigger on it that updates the date_modified column in the table, should I...

1) create a RULE in postgres

2) use the ON UPDATE (I saw an example of this, but couldn't find a really good example of how to do it.)

3) create a trigger.

create table foo ( 
  id primary key,
  date_modified date,
  other_data character(50)
);

I want the date_modified to change anytime anything else changes.