code: 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);
          }
      }
docs: Sets all the fields in one go from two instants representing an interval.
  <p>
  The chronology of the start instant is used, unless that is null when the
  chronology of the end instant is used instead.
  
  @param start  the start instant, null means now
  @param end  the end instant, null means now
  @throws ArithmeticException if the set exceeds the capacity of the period