Generate random timestamp.

        :param posix: POSIX time.
        :param kwargs: Kwargs for :meth:`~Datetime.datetime()`.
        :return: Timestamp.

def timestamp(self, posix: bool = True, **kwargs) -> Union[str, int]:
        """Generate random timestamp.

        :param posix: POSIX time.
        :param kwargs: Kwargs for :meth:`~Datetime.datetime()`.
        :return: Timestamp.
        """
        stamp = self.datetime(**kwargs)

        if posix:
            return timegm(stamp.utctimetuple())

        return stamp.strftime('%Y-%m-%dT%H:%M:%SZ')