code: public D plus(CalendarDays days) {
  
          long result = Math.addExact(this.getDaysSinceEpochUTC(), days.getAmount());
  
          try {
              return this.getChronology().getCalendarSystem().transform(result);
          } catch (IllegalArgumentException iae) {
              ArithmeticException ex = new ArithmeticException("Out of range: " + result);
              ex.initCause(iae);
              throw ex;
          }
  
      }
docs: /*[deutsch]
  <p>Addiert die angegebenen Kalendertage zu dieser Instanz. </p>
  
  @param   days    calendar days to be added
  @return  result of addition
  @throws  ArithmeticException in case of numerical overflow
  @since   3.4/4.3