code: public PeriodDuration plus(TemporalAmount amountToAdd) {
          PeriodDuration other = PeriodDuration.from(amountToAdd);
          return of(period.plus(other.period), duration.plus(other.duration));
      }
docs: Returns a copy of this amount with the specified amount added.
  <p>
  The parameter is converted using {@link PeriodDuration#from(TemporalAmount)}.
  The period and duration are combined separately.
  <p>
  This instance is immutable and unaffected by this method call.
  
  @param amountToAdd  the amount to add, not null
  @return a {@code Days} based on this instance with the requested amount added, not null
  @throws DateTimeException if the specified amount contains an invalid unit
  @throws ArithmeticException if numeric overflow occurs