protected function toDateTime(string $representation): DateTimeImmutable
    {
        // Due to a Tikkie bug, the API may return epoch timestamps with milliseconds instead of a ISO-8601 formatted string.
        // I reported this on 24-04-2019.
        if (is_numeric($representation)) {
            // Remove milliseconds and prepend with @ to mark as timestamp.
            $representation = '@'.substr($representation, 0, 10);
        }

        return new DateTimeImmutable($representation);
    }