docs: public void setPeriod(ReadableInstant start, ReadableInstant end) {
          if (start == end) {
              setPeriod(0L);
          } else {
              long startMillis = DateTimeUtils.getInstantMillis(start);
              long endMillis = DateTimeUtils.getInstantMillis(end);
              Chronology chrono = DateTimeUtils.getIntervalChronology(start, end);
              setPeriod(startMillis, endMillis, chrono);
          }
      }