std::string stime() {
        std::chrono::system_clock::time_point now = std::chrono::system_clock::now();
        auto now_c = std::chrono::system_clock::to_time_t(now);
        auto s = std::put_time(std::localtime(&now_c), "%F %T");
        std::stringstream ss;
        ss << s;
        return ss.str();
    }