Index: sites/all/modules/contrib/date/date.field.inc
===================================================================
--- sites/all/modules/contrib/date/date.field.inc	(date 1325770983000)
+++ sites/all/modules/contrib/date/date.field.inc	(revision )
@@ -327,6 +327,9 @@
     foreach ($items as $delta => $item) {
       if (is_array($item) && isset($item['value'])) {
         $process = date_process_values($field, $instance);
+        if (empty($item['timezone'])) {
+          $item['timezone'] = NULL;
+        }
         $date1 = new DateObject($item['value'], $item['timezone'], date_type_format($field['type']));
         if (count($process) == 1 || (empty($item['value2']) && $item['value2'] !== 0)) {
           $date2 = clone($date1);