def convert_timestamp(ts):
    """Converts the timestamp to a format suitable for Billing.

    Examples of a good timestamp for startTime, endTime, and eventTime:
        '2016-05-20T00:00:00Z'
    Note the trailing 'Z'. Python does not add the 'Z' so we tack it on
    ourselves.
    """
    return ts.isoformat() + 'Z'