code: private void appendISO8601(StringBuilder sb, long millis) {
          GregorianCalendar cal = new GregorianCalendar();
          cal.setTimeInMillis(millis);
          sb.append(cal.get(Calendar.YEAR));
          sb.append('-');
          a2(sb, cal.get(Calendar.MONTH) + 1);
          sb.append('-');
          a2(sb, cal.get(Calendar.DAY_OF_MONTH));
          sb.append('T');
          a2(sb, cal.get(Calendar.HOUR_OF_DAY));
          sb.append(':');
          a2(sb, cal.get(Calendar.MINUTE));
          sb.append(':');
          a2(sb, cal.get(Calendar.SECOND));
      }
docs: Append the time and date in ISO 8601 format