help on to_date conversion please....

I am converting an 8 digit number to a date. I don't see a postgres
function to do it direct, but two functions seems like I'm on the right
track, to_char and to_date. Individually I get them to work, but when I
combine them I get the wrong answer. see below. Can anyone help in
getting the right date from an 8 digit number input formatted as yyyymmdd?

hwds=# select to_char(20020101, '99999999');
to_char
-----------
20020101 CORRECT.
(1 row)

hwds=# select to_date('20020101','yyyymmdd');
to_date
------------
2002-01-01 CORRECT.
(1 row)

hwds=# select to_date(to_char(20020101, '99999999'),'yyyymmdd');
to_date
------------
2003-08-12 WHY DOES IT GIVE ME DIFFERENT ANSWER WHEN I COMBINE THE
FUNCTIONS?
(1 row)