THX - this works. But for German date format i use now this code:
defaultDateFormat: { 
validator: function(value, param){ 
 //consoleDebug(value);
 var re = /^\d{1,2}\.\d{1,2}\.\d{4}$/;
 if( re.test(value)){
 	var adata = value.split('.');
        var dd = parseInt(adata[0],10);
    	var mm = parseInt(adata[1],10);
    	var yyyy = parseInt(adata[2],10);
    	var xdata = new Date(yyyy,mm-1,dd);
    	if ( ( xdata.getFullYear() == yyyy ) && ( xdata.getMonth () == mm - 1 ) && ( xdata.getDate() == dd ) )
    		return true;
    	else
    		return false;
    	}
    	return false;        	
  } 
} 
}	
This code always checked dates like the "30. February" or other not valid dates.
I found that exsample on this site: 
http://snipplr.com/view/58062/