std::chrono::time_point::time_since_epoch(3)

NAME
std::chrono::time_point::time_since_epoch - std::chrono::time_point::time_since_epoch

Synopsis
   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