code: public static String dateTimeToString(DateTime input, String format, String timezone) {
      String tz = StringUtils.defaultString(timezone, ConfigurationKeys.DEFAULT_SOURCE_TIMEZONE);
      DateTimeZone dateTimeZone = getTimeZone(tz);
      DateTimeFormatter outputDtFormat = DateTimeFormat.forPattern(format).withZone(dateTimeZone);
      return outputDtFormat.print(input);
    }
docs: Convert joda time to a string in the given format
  @param input timestamp
  @param format expected format
  @param timezone time zone of timestamp
  @return string format of timestamp