def timesince(start_time, end_time, default="1天"):
    """
    Returns string representing "time since" e.g.
    3 days ago, 5 hours ago etc.
    """
    diff = end_time - start_time
    if end_time > start_time:
        periods = (
            (diff.days / 365, "年"),
            (diff.days / 30, "个月"),
#            (diff.days / 7, "周"),
            (diff.days, "天"),
#            (diff.seconds / 3600, "小时"),
#            (diff.seconds / 60, "分钟"),
#            (diff.seconds, "秒"),
        )
        for period, unit in periods:
            if period:
                return "%d%s" % (period, unit)

    return default