--- /wp-includes/default-widgets.php	Fri Jun 14 18:22:51 2013
+++ /wp-includes/default-widgets-patched.php	Fri Jun 14 18:23:04 2013
@@ -596,7 +596,7 @@
 		$instance = $old_instance;
 		$instance['title'] = strip_tags($new_instance['title']);
 		$instance['number'] = (int) $new_instance['number'];
-		$instance['show_date'] = (bool) $new_instance['show_date'];
+		$instance['show_date'] = isset( $new_instance['show_date'] ) ? (bool) $new_instance['show_date'] : false;
 		$this->flush_widget_cache();
 
 		$alloptions = wp_cache_get( 'alloptions', 'options' );