def reset_subscription_since(self, account_id, datetime_str):
        """Handler for `--reset-subscription-since` command.

        Args:
            account_id(int): id of the account to reset.
            datetime_str(str): string representing the datetime used in the
                next poll to retrieve data since.

        Returns:
            (str) json encoded response.

        NOTES:
            We don't care about validation here, we demand the responsibility to
            the backend.
        """
        data = {
            'account_id': account_id,
            'datetime': datetime_str,
        }
        return self._perform_post_request(self.reset_subscription_since_endpoint, data, self.token_header)