# std::chrono::time_point<Clock,Duration>::time_since_epoch

< cpp‎ | chrono‎ | time point

duration time_since_epoch() const; |  | (C++11 起) (C++14 前)
constexpr duration time_since_epoch() const; |  | (C++14 起)

返回表示 *this 与 `clock` 的纪元间的时间量的 duration 。

### 参数

（无）

### 返回值

此 `time_point` 与 `clock` 的纪元间的时间量

### 示例

    #include <iostream>
    #include <chrono>
    #include <ctime>

    int main()
    {
        auto p0 = std::chrono::time_point<std::chrono::system_clock>{};
        auto p1 = std::chrono::system_clock::now();
        auto p2 = p1 - std::chrono::hours(24);

        auto epoch_time = std::chrono::system_clock::to_time_t(p0);
        std::cout << "epoch: " << std::ctime(&epoch_time);
        auto today_time = std::chrono::system_clock::to_time_t(p1);
        std::cout << "today: " << std::ctime(&today_time);

        std::cout << "hours since epoch: "
                  << std::chrono::duration_cast<std::chrono::hours>(
                       p1.time_since_epoch()).count()
                  << '\n';
        std::cout << "yesterday, hours since epoch: "
                  << std::chrono::duration_cast<std::chrono::hours>(
                       p2.time_since_epoch()).count()
                  << '\n';
    }

可能的输出：

    epoch: Thu Jan  1 00:00:00 1970
    today: Fri Jun 30 10:44:11 2017
    hours since epoch: 416338
    yesterday, hours since epoch: 416314