Index: date/date_elements.inc
===================================================================
RCS file: /cvs/drupal-contrib/contributions/modules/date/date/date_elements.inc,v
retrieving revision 1.46.2.2.2.68
diff -u -r1.46.2.2.2.68 date_elements.inc
--- date/date_elements.inc	12 Aug 2010 21:31:07 -0000	1.46.2.2.2.68
+++ date/date_elements.inc	4 Oct 2010 08:56:08 -0000
@@ -515,6 +515,7 @@
         $errors[] = t('The To date is invalid.');
       }
       if (empty($errors)) {
+        if ($to_date_empty) $item[$to_field] = null;
         form_set_value($element, $item, $form_state);
       }
     }