code: public Interval withTimes(LocalTime startTime, LocalTime endTime) {
          requireNonNull(startTime);
          requireNonNull(endTime);
          return new Interval(this.startDate, startTime, this.endDate, endTime, this.zoneId);
      }
docs: Returns a new interval based on this interval but with a different start
  and end time.
  
  @param startTime the new start time
  @param endTime the new end time
  @return a new interval