code: @Override
      public ChronoPeriod plus(TemporalAmount amountToAdd) {
          ChronoPeriodImpl amount = validateAmount(amountToAdd);
          return new ChronoPeriodImpl(
                  chrono,
                  Math.addExact(years, amount.years),
                  Math.addExact(months, amount.months),
                  Math.addExact(days, amount.days));
      }
docs: -----------------------------------------------------------------------