Date Type

Can anyone help me with the Date type?  I can't seem to insert null values
into a
field I created of type Date.  I am trying to import from a flat file made
from
an ancient business system.  It stores the ship date as 0000-00-00 for items
that
have not shipped.  This imports fine into mysql and mssql, but does not
import into
postgresql.  I have also tried just putting a blank field when these is a
0000-00-00,
but that also does not seem to work. The data file goes something like this:

order number;order date;ship date;
000000000001;2001-01-01;0000-00-00;
000000000002;2001-01-01;2001-02-01;
...

If any more info is needed please ask, I am not really sure what all is
needed.  Any
help is definetly appriciated.

Thanks,
Mark