code: void formatEra(StringBuilder b, ZonedDateTime d, int width, FieldVariants eras) {
      int year = d.getYear();
      int index = year < 0 ? 0 : 1;
      switch (width) {
        case 5:
          b.append(eras.narrow[index]);
          break;
  
        case 4:
          b.append(eras.wide[index]);
          break;
  
        case 3:
        case 2:
        case 1:
          b.append(eras.abbreviated[index]);
          break;
      }
    }
docs: Format the era based on the year.