help on a function with exception

Hi,

I want to do a check on a column if other columns meet certain 
conditions. The program I'm working with allows to create additional 
columns on every 'object' - called extra data, but I have no control 
over the program.  I want to enforce the values on this one extra data 
to be of type date.

My idea was to do a Trigger function and cast to a date and if there's 
an exception, raise an error.  Below is what I've tried, but it just 
keeps on Raising Exception.

Could someone please help me? The date I enter is: 2012-10-10 which 
works fine if I do a:
select '2012-10-10'::date

Thanks

--Postgres 9.0

CREATE OR REPLACE FUNCTION fnc_check_PO_extra_date()  RETURNS trigger AS
$BODY$
DECLARE    tmp_date date;
BEGIN  IF (TG_OP = 'INSERT' OR TG_OP = 'UPDATE') THEN    IF (NEW.data_value IS NOT NULL or new.data_value = '') and 
NEW.extra_id =        (select extra_id from extra_data where data_type = 9            and (data_name = 'ETA' or
data_name= 'Adjusted ETA'))    THEN        tmp_date := new.data_value::date;    END IF;  END IF;  EXCEPTION    WHEN
othersTHEN    RAISE EXCEPTION 'Invalid date on Extra Data!';
 
return NEW;
END;
$BODY$
LANGUAGE plpgsql VOLATILE

CREATE TRIGGER trg_check_PO_extra_date  BEFORE INSERT OR UPDATE  ON extra_values  FOR EACH ROW  EXECUTE PROCEDURE
fnc_check_PO_extra_date();