code: public DateTimeFormatterBuilder appendFixedSignedDecimal(
              DateTimeFieldType fieldType, int numDigits) {
          if (fieldType == null) {
              throw new IllegalArgumentException("Field type must not be null");
          }
          if (numDigits <= 0) {
              throw new IllegalArgumentException("Illegal number of digits: " + numDigits);
          }
          return append0(new FixedNumber(fieldType, numDigits, true));
      }
docs: Instructs the printer to emit a field value as a fixed-width decimal
  number (smaller numbers will be left-padded with zeros), and the parser
  to expect an signed decimal number with the same fixed width.
  
  @param fieldType  type of field to append
  @param numDigits  the exact number of digits to parse or print, except if
  printed value requires more digits
  @return this DateTimeFormatterBuilder, for chaining
  @throws IllegalArgumentException if field type is null or if <code>numDigits <= 0</code>
  @since 1.5