def _build_str_from_expression(exp_items):
    """
    根据解析出的时间表达式的关键字计算对应的表达式值
    :return: 表达式对应时间的标准时间格式字符串表示形式
    """
    time_delta_base, unit, method = exp_items
    if unit not in _KEY_TIME_MAPPING:
        return None
    delta = datetime.timedelta(seconds=int(time_delta_base) * _KEY_TIME_MAPPING[unit])
    now = datetime.datetime.now()
    return datetime.datetime.strftime(now + (delta, -delta)[u'前' == method], STARDAND_FORMAT)