from django.contrib.admin.filters import DateFieldListFilter

class DateFieldListFilterOrNull(DateFieldListFilter):
    def __init__(self, field, request, params, model, model_admin, field_path):
        self.lookup_kwarg_isnull = '%s__isnull' % (field_path, )
        super(DateFieldListFilterOrNull, self).__init__(field, request, params, model, model_admin, field_path)
        self.links = self.links + (
           ('None', {
                self.lookup_kwarg_isnull: 'True',
            }),
           ('Not none', {
                self.lookup_kwarg_isnull: 'False',
            }),
           )

    def expected_parameters(self):
        return super(DateFieldListFilterOrNull, self).expected_parameters() + [self.lookup_kwarg_isnull, ]