code: public static String iso8601(Period period, Duration duration) {
          Objects.requireNonNull(period, "period must not be null");
          Objects.requireNonNull(duration, "duration must not be null");
          if (period.isZero()) {
              return duration.toString();
          }
          if (duration.isZero()) {
              return period.toString();
          }
          return period.toString() + duration.toString().substring(1);
      }
docs: Formats a period and duration to a string in ISO-8601 format.
  <p>
  To obtain the ISO-8601 format of a {@code Period} or {@code Duration}
  individually, simply call {@code toString()}.
  See also {@link PeriodDuration}.
  
  @param period  the period to format
  @param duration  the duration to format
  @return the ISO-8601 format for the period and duration