code: static long transform(
          int relgregyear,
          int dayOfYear
      ) {
  
          if (relgregyear >= 1873) {
              return PlainDate.of(relgregyear, dayOfYear).getDaysSinceEpochUTC();
          } else {
              return START_OF_YEAR[relgregyear - 701] + dayOfYear - 1;
          }
  
      }
docs: also used in serialization