--- date/date/date_elements.inc	2009-05-14 16:58:00.000000000 +0200
+++ date/date/date_elements.inc	2009-06-18 20:45:01.000000000 +0200
@@ -462,6 +462,7 @@
         $errors[] = t('Some value must be entered in the To date.');
       }

+      // check if field is empty and default_value was set to blank 
+      $blank_to_fix = $to_date_empty && $field['widget']['default_value2'] === 'blank';
       $element[$to_field]['#value'] = $merged_date;
       
       // Call the right function to turn this altered user input into
@@ -525,6 +526,9 @@
         $errors[] = t('The To date is invalid.');
       }
       if (empty($errors)) {
+        if ($blank_to_fix)
+            $item[$to_field] = NULL;
+
         form_set_value($element, $item, $form_state);
       }
     }