Debugging pgsql function date error

I have a long plpgsql function which takes a date as one of its
arguments but I am receiving an error for another date! I'm using 8.3 on
Debian.

CREATE OR REPLACE FUNCTION
fn_alert01_maker(integer, integer, integer, integer, date, integer)
RETURNS SETOF alert_info
AS $$
DECLARE
userid ALIAS for $1;
alert ALIAS for $2;
since ALIAS for $3;
wait ALIAS for $4;
date_from ALIAS for $5;
days_until ALIAS for $6;
resulter alert_info%rowtype;
since_int INTERVAL;
wait_int INTERVAL;
days_int INTERVAL;

BEGIN

days_int := days_until || ' days';
since_int := since || ' seconds';
wait_int := wait || ' seconds';
...

and has that argument as one of its WHERE conditions:
...
WHERE
rs.dt_modified > ((now() - wait_int) - since_int)
AND
rs.d_date >= date_from
AND
rs.d_date <= date_from + days_int
...

Wierdly, if I call it like this, it is fine:
SELECT * FROM fn_alert01_maker(0, 1, 845, 5, '2009-05-31', 5);

But like this it throws an error:

test=> SELECT * FROM fn_alert01_maker(0, 1, 8450, 5, '2009-05-31', 5);
ERROR: invalid input syntax for integer: "2009-06-01"
CONTEXT: PL/pgSQL function "fn_alert01_maker" line 30 at FOR over
SELECT rows

Any ideas?

Rory