code: public void setPeriod(ReadableInterval interval) {
          if (interval == null) {
              setPeriod(0L);
          } else {
              Chronology chrono = DateTimeUtils.getChronology(interval.getChronology());
              setPeriod(interval.getStartMillis(), interval.getEndMillis(), chrono);
          }
      }
docs: Sets all the fields in one go from an interval using the ISO chronology
  and dividing the fields using the period type.
  
  @param interval  the interval to set, null means zero length
  @throws ArithmeticException if the set exceeds the capacity of the period