--- Date.php.orig	2012-07-10 19:14:26.000000000 +0200
+++ Date.php	2012-07-10 19:16:22.000000000 +0200
@@ -627,16 +627,25 @@
             return;
         }
 
-        $this->_mday = 1;
-        $first = $this->dayOfWeek();
-        if ($weekday < $first) {
-            $this->_mday = 8 + $weekday - $first;
-        } else {
-            $this->_mday = $weekday - $first + 1;
+        if ($nth < 0) {  // last $weekday of month
+            $this->_mday = $lastday = Horde_Date_Utils::daysInMonth($this->_month, $this->_year);
+            $last = $this->dayOfWeek();
+            $this->_mday += ($weekday - $last);
+            if ($this->_mday > $lastday)
+                $this->_mday -= 7;
+        }
+        else {
+            $this->_mday = 1;
+            $first = $this->dayOfWeek();
+            if ($weekday < $first) {
+                $this->_mday = 8 + $weekday - $first;
+            } else {
+                $this->_mday = $weekday - $first + 1;
+            }
+            $diff = 7 * $nth - 7;
+            $this->_mday += $diff;
+            $this->_correct(self::MASK_DAY, $diff < 0);
         }
-        $diff = 7 * $nth - 7;
-        $this->_mday += $diff;
-        $this->_correct(self::MASK_DAY, $diff < 0);
     }
 
     /**