private static String convertTimestamp(long ms) {
    calendar.setTimeInMillis(ms);

    int hrs = calendar.get(Calendar.HOUR_OF_DAY);
    int mins = calendar.get(Calendar.MINUTE);
    int secs = calendar.get(Calendar.SECOND);

    NumberFormat nf = NumberFormat.getIntegerInstance();
    nf.setMinimumIntegerDigits(2);
    return nf.format(hrs) + ":" + nf.format(mins) + ":" + nf.format(secs);
  }