DateTimeField fromDate = new DateTimeField("fromDate"){

   private static final long serialVersionUID = 1L;

        @Override
        protected DatePicker newDatePicker() {
            DatePicker datePicker = new DatePicker(){

                private static final long serialVersionUID = 1L;

                @Override
                protected String getDatePattern() {
                     return ((String) getSession().getAttribute("dateFormat")).replace("yyyy", "yy");
                }

         };
         return datePicker;
    }