Help with trigger

I'm new to PostgreSQL, but have worked with other databases. I'm trying to
write a trigger to default a timestamp column to a fixed interval before
another. The test setup is as follows:

create table test
( date1 timestamp,
date2 timestamp
);

create or replace function t_listing_startdate() returns trigger as
$t_listing_startdate$
begin
if NEW.date2 is null then
NEW.date2 := NEW.date1 - interval '7 day';
end if;
return NEW;
end;
$t_listing_startdate$ LANGUAGE plpgsql;

CREATE TRIGGER t_listing_startdate before insert or update on test
for each row execute procedure t_listing_startdate();

Insert into test(date1) values('May 4, 2012');
INSERT 0 1
test=# select * from test;
date1 | date2
---------------------+-------
2012-04-27 00:00:00 |
(1 row)

I'm obviously missing something ... and probably something obvious. Why is
date2 still null?

Thanks much
---Michael