function IsNumeric(strString)

   //  check for valid numeric strings    

   {

   var strValidChars = "0123456789.-";

   var strChar;

   var blnResult = true;



   if (strString.length == 0) return false;



   //  test strString consists of valid characters listed above

   for (i = 0; i < strString.length && blnResult == true; i++)

      {

      strChar = strString.charAt(i);

      if (strValidChars.indexOf(strChar) == -1)

         {

         blnResult = false;

         }

      }

   return blnResult;

   }



    function check_monat() {

    

        if (!document.all.bis_mm.value) return true;

        

        var num = IsNumeric(document.all.bis_mm.value);

        

        if((document.all.bis_mm.value > 12) || (document.all.bis_mm.value < 1) || (num == false)) {

            

            alert('Der Monat muss zwischen 1 und 12 liegen!');

            document.all.bis_mm.focus();

        }

        

    }

    

    function check_jahr() {

        

        if (!document.all.bis_jj.value) return true;

        

        var num = IsNumeric(document.all.bis_jj.value);

        

        if (num == false) {

            

            alert('Das Jahr muss numerisch sein!');

            document.all.bis_jj.focus();

        }

        

    }