code: private ZonedDateTime parseDateTime(MessageArg arg) {
      long instant = arg.asLong();
      ZoneId timeZone = arg.timeZone() != null ? arg.timeZone() : this.timeZone;
      return ZonedDateTime.ofInstant(Instant.ofEpochMilli(instant), timeZone);
    }
docs: Parse the argument as a date-time with the format-wide time zone.