The ever persistant typo.....

modules/calendar.module

@@ -30,7 +30,7 @@ function display() {
     // Calculate previous and next months dates and check for shorter months (28/30 days)
     $prevmonth = mktime(23, 59, 59, $month - 1, 1, $year);
     $prev = mktime(23, 59, 59, $month - 1, min(date("t", $prevmonth), $day), $year);
-    $nextmonth = mktime(23, 59, 59, $month + 1, 31, $year);
+    $nextmonth = mktime(23, 59, 59, $month + 1, 1, $year);
     $next = mktime(23, 59, 59, $month + 1, min(date("t", $nextmonth), $day), $year);
     
     // Generate calendar header: