diff --git a/clientside_validation.js b/clientside_validation.js
index 3cc334f..5ac0398 100644
--- a/clientside_validation.js
+++ b/clientside_validation.js
@@ -1147,7 +1147,13 @@
       var expectedpartscount = 0;
       var day = parseInt(parts[param.daypos], 10);
       var month = parseInt(parts[param.monthpos], 10);
-      month = month - 1;
+      if (isNaN(month)) {
+        month = new Date(parts[param.monthpos] + " 1, 2000");
+        month = month.getMonth();
+      }
+      else {
+        month = month - 1;
+      }
       var year = parseInt(parts[param.yearpos], 10);
       var date = new Date();
       var result = true;
@@ -1293,4 +1299,4 @@
     }
   };
 
-})(jQuery);
\ No newline at end of file
+})(jQuery);