diff --git a/sites/all/modules/signup_scheduler/includes/node_api.inc b/sites/all/modules/signup_scheduler/includes/node_api.inc
index e352806..38a5f1e 100644
--- a/sites/all/modules/signup_scheduler/includes/node_api.inc
+++ b/sites/all/modules/signup_scheduler/includes/node_api.inc
@@ -29,11 +29,15 @@ function signup_scheduler_nodeapi_update(&$node) {
 
   $timezone = date_default_timezone_name();
   $open = date_make_date($node->signup_scheduler_open, $timezone, DATE_ISO);
-  date_timezone_set($open, timezone_open('UTC'));
+  if (!empty($open)) {
+    date_timezone_set($open, timezone_open('UTC'));
+  }
   $open = date_convert($open, DATE_OBJECT, DATE_UNIX);
   
   $close = date_make_date($node->signup_scheduler_close, $timezone, DATE_ISO);
-  date_timezone_set($close, timezone_open('UTC'));
+  if (!empty($close)) {
+    date_timezone_set($close, timezone_open('UTC'));
+  }
   $close = date_convert($close, DATE_OBJECT, DATE_UNIX);
   
   db_query("INSERT INTO {signup_scheduler} (nid, status, open, close) VALUES (%d, %d, %d, %d)", $node->nid, $node->signup_scheduler_status, $open, $close);