code: public static PeriodDuration of(Period period, Duration duration) {
          Objects.requireNonNull(period, "The period must not be null");
          Objects.requireNonNull(duration, "The duration must not be null");
          return new PeriodDuration(period, duration);
      }
docs: Obtains an instance based on a period and duration.
  <p>
  The total amount of time of the resulting instance is the period plus the duration.
  
  @param period  the period, not null
  @param duration  the duration, not null
  @return the combined period-duration, not null