code: static InternationalFixedDate ofYearDay(int prolepticYear, int dayOfYear) {
          YEAR_RANGE.checkValidValue(prolepticYear, ChronoField.YEAR_OF_ERA);
          ChronoField.DAY_OF_YEAR.checkValidValue(dayOfYear);
  
          boolean isLeapYear = INSTANCE.isLeapYear(prolepticYear);
          int lastDoy = (DAYS_IN_YEAR + (isLeapYear ? 1 : 0));
          if (dayOfYear > lastDoy) {
              throw new DateTimeException("Invalid date 'DayOfYear 366' as '" + prolepticYear + "' is not a leap year");
          }
          if (dayOfYear == lastDoy) {
              return new InternationalFixedDate(prolepticYear, 13, 29);
          }
          if (dayOfYear == LEAP_DAY_AS_DAY_OF_YEAR && isLeapYear) {
              return new InternationalFixedDate(prolepticYear, 6, 29);
          }
          int doy0 = dayOfYear - 1;
          if (dayOfYear >= LEAP_DAY_AS_DAY_OF_YEAR && isLeapYear) {
              doy0--;
          }
          int month = (doy0 / DAYS_IN_MONTH) + 1;
          int day = (doy0 % DAYS_IN_MONTH) + 1;
          return new InternationalFixedDate(prolepticYear, month, day);
      }
docs: Obtains a {@code InternationalFixedDate} representing a date in the International fixed calendar
  system from the proleptic-year and day-of-year fields.
  <p>
  This returns a {@code InternationalFixedDate} with the specified fields.
  The day must be valid for the year, otherwise an exception will be thrown.
  
  @param prolepticYear  the International fixed proleptic-year
  @param dayOfYear  the International fixed day-of-year, from 1 to 366
  @return the date in International fixed calendar system, not null
  @throws DateTimeException if the value of any field is out of range,
  or if the day-of-year is invalid for the year