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

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C-style date and time
std::chrono::time_point
duration time_since_epoch ( ) const ;
(since C++11)
(constexpr since C++14)

Returns a duration representing the amount of time between * this and the clock 's epoch.

Parameters

(none)

Return value

The amount of time between this time_point and the clock 's epoch.

Example

Run this code 
#include <chrono>
#include <ctime>
#include <iostream>
 
int main()
{
    const auto p0 = std::chrono::time_point<std::chrono::system_clock>{};
    const auto p1 = std::chrono::system_clock::now();
    const auto p2 = p1 - std::chrono::hours(24);
 
    std::time_t epoch_time = std::chrono::system_clock::to_time_t(p0);
    std::cout << "epoch: " << std::ctime(&epoch_time);
    std::time_t 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';
}

Possible output: 

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
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