def _sort_time_windows_since(tw1, tw2):
    """
    Sort ascending two time windows based on their since values
    :param TimeWindow tw1: One of two time windows
    :param TimeWindow  tw2: One of two time windows
    :return: A list with sorted time windows
    :rtype: list(TimeWindow)
    """
    if tw1.since < tw2.since:
        return [tw1, tw2]
    else:
        return [tw2, tw1]