trigger conversion advice needed

I'm working on converting a simple trigger from Oracle to Postgres and I
have a couple ofl questions that I need some help on please. First here's
the Oracle trigger:

CREATE OR REPLACE TRIGGER t_ship_date
AFTER UPDATE OR INSERT OF order_date ON orders
BEGIN
UPDATE orders
SET ship_date = working_5days(order_date);
END;
/

When I was working on converting the trigger I noticed that Postgres doesn't
have the OF table_attribute ON syntax support. I just want the trigger to
fire when the order_date field in the order table is updated or inserted
like it is specified in the Oracle trigger specification above. So I did
this in Postgres:

CREATE OR REPLACE FUNCTION t_ship_date()
RETURNS TRIGGER AS '
BEGIN
UPDATE orders
SET ship_date = working_5days(new.order_date);

RETURN NEW;
END;
' LANGUAGE 'plpgsql';

I justed used new.order_date in the Postgres one version. I'm pretty sure
that this isn't going to work, it will probably update every ship_date which
is not what I want. I just want to update the ship_date when the record's
order_date is updated. Is there some way in Postgres to specify a trigger to
fire only when a certain field in a table is changed or inserted (like I had
in the Oracle version)? I couldn't find anything for triggers to do that in
the docs so I hope someone can shed some light on this for me. I thought
maybe something like this could work too (probably closer to being correct):

CREATE OR REPLACE FUNCTION t_ship_date()
RETURNS TRIGGER AS '
BEGIN
IF TG_OP = ''INSERT'' THEN
UPDATE orders
SET ship_date = working_5days(new.order_date)
where order_no = new.order_no;
ELSIF TG_OP =''UPDATE'' THEN
UPDATE orders
SET ship_date = working_5days(new.order_date)
where order_no = old.order_no;
END IF;

RETURN NEW;
END;
' LANGUAGE 'plpgsql';

The working_5days function just adds 5 business days to a date fyi. Now the
second question I have is due to an error I keep getting when I try and make
the trigger definition below:

CREATE TRIGGER t_ship_date AFTER UPDATE OR INSERT ON orders
EXECUTE PROCEDURE t_ship_date();

I always get a parse error at or near execute. What am I doing wrong? If I
could get the trigger to compile then I could test it to see if it works the
same as the Oracle version. I wouldn't even have to ask the first question
if I could get it to compile but I thought I might as well ask anyway so I
don't have to post another message. Thanks for the help.
-Clint

P.S. 7.3.4 is the postgres version on the server.

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