docs: protected static int between(ReadableInstant start, ReadableInstant end, DurationFieldType field) {
          if (start == null || end == null) {
              throw new IllegalArgumentException("ReadableInstant objects must not be null");
          }
          Chronology chrono = DateTimeUtils.getInstantChronology(start);
          int amount = field.getField(chrono).getDifference(end.getMillis(), start.getMillis());
          return amount;
      }