code: public PlainDate convert(HistoricDate date) {
  
          if (this.isOutOfRange(date)) {
              throw new IllegalArgumentException("Out of supported range: " + date);
          }
  
          Calculus algorithm = this.getAlgorithm(date);
  
          if (algorithm == null) {
              throw new IllegalArgumentException("Invalid historic date: " + date);
          }
  
          return PlainDate.of(algorithm.toMJD(date), EpochDays.MODIFIED_JULIAN_DATE);
  
      }
docs: /*[deutsch]
  <p>Konvertiert das angegebene historische Datum zu einem ISO-8601-Datum. </p>
  
  @param   date    historic calendar date
  @return  ISO-8601-date (gregorian)
  @throws  IllegalArgumentException if given date is invalid or out of supported range
  @see     #isValid(HistoricDate)
  @since   3.0