code: public long getLengthInDays() {
  
          if (this.isFinite()) {
              long days = CalendarUnit.DAYS.between(
                  this.getStart().getTemporal(),
                  this.getEnd().getTemporal());
              if (this.getStart().isOpen()) {
                  days--;
              }
              if (this.getEnd().isClosed()) {
                  days++;
              }
              return days;
          } else {
              throw new UnsupportedOperationException(
                  "An infinite interval has no finite duration.");
          }
  
      }
docs: /*[deutsch]
  <p>Liefert die L&auml;nge dieses Intervalls in Tagen. </p>
  
  @return  duration in days as long primitive
  @throws  UnsupportedOperationException if this interval is infinite
  @since   2.0
  @see     #getDurationInYearsMonthsDays()
  @see     #getDuration(CalendarUnit[]) getDuration(CalendarUnit...)