def iter_sort_response(response):
    """
    Sorts the VSO query results by their start time.

    Parameters
    ----------
    response : `zeep.objects.QueryResponse`
        A SOAP Object of a VSO query result

    Returns
    -------
    `list`
        Sorted record items w.r.t. their start time.
    """
    has_time_recs = list()
    has_notime_recs = list()
    for prov_item in response.provideritem:
        if not hasattr(prov_item, 'record') or not prov_item.record:
            continue
        if not hasattr(prov_item.record, 'recorditem') or not prov_item.record.recorditem:
            continue
        rec_item = prov_item.record.recorditem
        for rec in rec_item:
            if hasattr(rec, 'time') and hasattr(rec.time, 'start') and rec.time.start is not None:
                has_time_recs.append(rec)
            else:
                has_notime_recs.append(rec)
    has_time_recs = sorted(has_time_recs, key=lambda x: x.time.start)
    all_recs = has_time_recs + has_notime_recs
    return all_recs