I have yet to see one single bug with this version of phpBB2.
The Birthday syntax used was
d-m-Y (when you tried this)
in your case.. 11-01-1983 (11th day of Jan. 1983)

.. I guess the programmer is European.
I recoded it though so you should be able to enter it normally now.
The Birthday syntax used
now is
m-d-Y
Hope this helps.