diff -Naur WebCalendar-1.2.0.orig/includes/functions.php WebCalendar-1.2.0.new/includes/functions.php
--- WebCalendar-1.2.0.orig/includes/functions.php	2009-01-26 12:57:29.000000000 +0100
+++ WebCalendar-1.2.0.new/includes/functions.php	2009-01-26 13:06:09.000000000 +0100
@@ -2293,6 +2293,14 @@
     $byxxxDay = '';
     $dayTxt = substr ( $day, -2, 2 );
     $dayOffset = substr_replace ( $day, '', -2, 2 );
+
+    // It is possible to have spurious offset days within a 'daily' repetition,
+    //   by setting them while in month/year repetition type, then changing
+    //   type to 'daily'.
+    // These situations will lead in a crash without the following test.
+    if (is_numeric($dayOffset) && !isset($ditype))
+      continue;
+
     $dowOffset = ( ( -1 * $byday_values[$dayTxt] ) + 7 ) % 7; //SU=0, MO=6, TU=5...
     if ( is_numeric ( $dayOffset ) && $dayOffset > 0 ) {
       // Offset from beginning of $type.