code: 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);
      }
docs: Creates a new interval with the specified period after the start instant.
  
  @param period  the period to add to the start to get the new end instant, null means zero
  @return an interval with the start from this interval and a calculated end
  @throws IllegalArgumentException if the period is negative