code: @Override
      protected PlainDate getDate0(int year) {
  
          int month = this.getMonth().getValue();
          int dayOfMonth = GregorianMath.getLengthOfMonth(year, month) + 1 + this.domIndicator;
          int ref = GregorianMath.getDayOfWeek(year, month, dayOfMonth);
          PlainDate result = PlainDate.of(year, month, dayOfMonth);
  
          if (ref == this.dayOfWeek) {
              return result;
          }
  
          int delta = (this.dayOfWeek - ref);
  
          if (delta > 0) {
              delta -= 7;
          }
  
          return result.plus(delta, DAYS);
  
      }
docs: ~ Methoden ----------------------------------------------------------