Determining if VARCHAR2 column contains valid date

I need to determine if a VARCHAR2 column contains a valid date. Is there a function to test if the value in the column is a valid date including leap years? Example: A_Date varchar2(8)

A_Date    Valid
--------  -----
20030131  Yes
20030631  No

The function TO_DATE can determine if a string represents a valid date. The problem with TO_DATE is that it won't tell you politely when the string is not a valid date: it will raise an error instead. How can you keep it from raising an error? EXCEPTION:

CREATE OR REPLACE FUNCTION  to_dt
(
    in_dt_text   IN  VARCHAR2,
    in_fmt_text  IN  VARCHAR2
                 DEFAULT 'YYYYMMDD'
)
RETURN  DATE
IS
BEGIN
    RETURN  TO_DATE ( in_dt_text,
                      in_fmt_text
                    );
EXCEPTION
    WHEN OTHERS
    THEN
        RETURN  NULL;
END   to_dt;
/

SELECT  a_date,
        NVL2 ( to_dt (a_date),
               'Yes',
               'No'
             )    AS valid
FROM    a_table;

If I really hated having that messy NVL2 call in my code, I would hide it (or rather its equivalent, since PL/SQL doesn't support NVL2 yet) in another function:

CREATE OR REPLACE FUNCTION  to_dt_yes_no
(
    in_dt_text   IN  VARCHAR2,
    in_fmt_text  IN  VARCHAR2
                 DEFAULT  'YYYYMMDD'
)
RETURN  VARCHAR2
IS
BEGIN
    IF  to_dt ( in_dt_text,
                in_fmt_text
              )  IS NULL
    THEN
        RETURN  'No';
    ELSE
        RETURN  'Yes';
    END IF;
END  to_dt_yes_no;
/

SELECT  a_date,
        to_dt_yes_no (a_date)
            AS valid
FROM    a_table;