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