date function not recognized :(

[yxez]

Hi Friends,

I am doing this simple select command against my table EMP...

select count(*) from EMP where
to_date(birthdate,'mmddyyyy')>='01-JAN-90';

to_date(birthdate,'mmddyyyy')>='01-JAN-90'
*
ERROR at line 2:
ORA-01843: not a valid month


***birthdate column is varchar2(8) of format 'mmddyyyy' (to allow conversion errors loaded from
foxpro database)
To bypass erroneous dates, I created this function:
=====================
CREATE OR REPLACE FUNCTION is_date (p_string VARCHAR2)
RETURN INTEGER
AS
v_dummy DATE;
BEGIN
v_dummy := TO_DATE(p_string, 'MMDDYYYY');
RETURN 0;
EXCEPTION
WHEN OTHERS THEN RETURN 1;
END;
/
======================

Then I run my second program.....

select count(*) from EMP where
to_date(birthdate,'mmddyyyy')>='01-JAN-90' AND IS_DATE(BIRTHDATE)=0;

to_date(birthdate,'mmddyyyy')>='01-JAN-90' AND IS_DATE(BIRTHDATE)=0
*
ERROR at line 2:
ORA-01843: not a valid month


....WHY IS IT THAT MY FUNCTION "IS_DATE" NOT WORKING?


THANKS A LOT