Unexpected parse behaviour for date to timestamp conversion

I have this field:

submit   | timestamp(0) without time zone


The following two SQL queries return different values depending on
wether the date is quoted or not ...

I am assuming it is because the date is being parsed differently.

# select submit from invoices where submit <= 2003-03-09;
  submit
----------
(0 rows)

# select submit from invoices where submit <= '2003-03-09';
  submit
---------------------
  2003-03-08 00:24:25
(1 row)


Why is the '2003-03-09' parsed/unserstood differently than 2003-03-09?

Thanks,

Jc