docs: public Interval withDurationAfterStart(ReadableDuration duration) {
          long durationMillis = DateTimeUtils.getDurationMillis(duration);
          if (durationMillis == toDurationMillis()) {
              return this;
          }
          Chronology chrono = getChronology();
          long startMillis = getStartMillis();
          long endMillis = chrono.add(startMillis, durationMillis, 1);
          return new Interval(startMillis, endMillis, chrono);
      }