docs: public Interval withPeriodAfterStart(ReadablePeriod period) {
          if (period == null) {
              return withDurationAfterStart(null);
          }
          Chronology chrono = getChronology();
          long startMillis = getStartMillis();
          long endMillis = chrono.add(period, startMillis, 1);
          return new Interval(startMillis, endMillis, chrono);
      }