:return: When the document was created
        :rtype: :py:class:`datetime.datetime`

def created_at(self):
        """
        :return: When the document was created
        :rtype: :py:class:`datetime.datetime`
        """
        if self._created_at:
            return self._created_at
        elif not self.abstract:
            return self.read_meta()._created_at

        raise EmptyDocumentException()