code: static InternationalFixedDate ofEpochDay(long epochDay) {
          EPOCH_DAY_RANGE.checkValidValue(epochDay, ChronoField.EPOCH_DAY);
          long zeroDay = epochDay + DAYS_0000_TO_1970;
  
          // The two values work great for any dates, just not the first (N/01/01) or the last of the year (N/0/0).
          long year = (400 * zeroDay) / DAYS_PER_CYCLE;
          long doy = zeroDay - (DAYS_IN_YEAR * year + InternationalFixedChronology.getLeapYearsBefore(year));
  
          boolean isLeapYear = INSTANCE.isLeapYear(year);
  
          // In some cases, N/01/01 (January 1st) results in (N-1)/0/0, i.e. -1 day off.
          if (doy == (DAYS_IN_YEAR + 1) && !isLeapYear) {
              year += 1;
              doy = 1;
          }
  
          // In some cases, N/0/0 results in (N+1)/0/0 (rubbish), in a way +1 year off.
          if (doy == 0) {
              year -= 1;
              doy = DAYS_IN_YEAR + (isLeapYear ? 1 : 0);
          }
  
          return ofYearDay((int) year, (int) doy);
      }
docs: Obtains a {@code InternationalFixedDate} representing a date in the International fixed calendar
  system from the epoch-day.
  
  @param epochDay  the epoch day to convert based on 1970-01-01 (ISO)
  @return the date in International fixed calendar system, not null
  @throws DateTimeException if the epoch-day is out of range