code: public int lengthOfMonth() {
  
          try {
              PlainDate min = this.with(DAY_OF_MONTH, this.getMinimum(DAY_OF_MONTH).intValue()).gregorian;
              PlainDate max = this.with(DAY_OF_MONTH, this.getMaximum(DAY_OF_MONTH).intValue()).gregorian;
              return ((int) CalendarDays.between(min, max).getAmount()) + 1;
          } catch (RuntimeException re) {
              return -1; // only in very exotic circumstances
          }
  
      }
docs: /*[deutsch]
  <p>Liefert die L&auml;nge des aktuellen historischen Monats in Tagen. </p>
  
  @return  length of historic month in days or {@code -1} if the length cannot be determined