code: @Override
      public ChronoPeriod plus(TemporalAmount amountToAdd) {
          if (amountToAdd instanceof ChronoPeriodImpl) {
              ChronoPeriodImpl amount = (ChronoPeriodImpl) amountToAdd;
              if (amount.getChronology().equals(getChronology())) {
                  return new ChronoPeriodImpl(
                          chronology,
                          Jdk8Methods.safeAdd(years, amount.years),
                          Jdk8Methods.safeAdd(months, amount.months),
                          Jdk8Methods.safeAdd(days, amount.days));
              }
          }
          throw new DateTimeException("Unable to add amount: " + amountToAdd);
      }
docs: -------------------------------------------------------------------------