code: public PlainDate getGregorianCutOverDate() {
  
          long start = this.events.get(this.events.size() - 1).start;
  
          if (start == Long.MIN_VALUE) {
              throw new UnsupportedOperationException("Proleptic history without any gregorian reform date.");
          }
  
          return PlainDate.of(start, EpochDays.MODIFIED_JULIAN_DATE);
  
      }
docs: /*[deutsch]
  <p>Liefert das Datum der letztlichen Einf&uuml;hrung des gregorianischen Kalenders. </p>
  
  @return  ISO-8601-date (gregorian)
  @throws  UnsupportedOperationException if this history is proleptic
  @see     #hasGregorianCutOverDate()
  @since   3.0