Help with trigger

[Michael Satterwhite <michael@xxxxxxxxxxx>]

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

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