code: static void printNanos(
          StringBuilder sb,
          int nano
      ) {
  
          sb.append(PlainTime.ISO_DECIMAL_SEPARATOR);
          String num = Integer.toString(nano);
          int len;
  
          if ((nano % MIO) == 0) {
              len = 3;
          } else if ((nano % KILO) == 0) {
              len = 6;
          } else {
              len = 9;
          }
  
          for (int i = num.length(); i < 9; i++) {
              sb.append('0');
          }
  
          for (int i = 0, n = len + num.length() - 9; i < n; i++) {
              sb.append(num.charAt(i));
          }
  
      }
docs: also called by ZonalDateTime