It is actually not the dashes, but the 0 (zeros):
testdb=# select to_date('2001-3-6 10:00', 'YYYY-FMMM-FMDD HH24:MI');
to_date
------------
2001-03-06
(1 row)
testdb=# select to_date('2001-12-16 10:00', 'YYYY-FMMM-FMDD HH24:MI');
to_date
------------
2001-12-16
(1 row)