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();
        }
        
    }