def billing_period_end_date(d, i, r):
    """Get billing period start date

    Using the stop of the report

    :param d: Report definition
    :type d: Dict
    :param i: Item definition
    :type i: Dict
    :param r: Meter reading
    :type r: usage.reading.Reading
    :return: isoformatted stop date
    :rtype: String
    """
    return r.stop.isoformat()