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Showing content from https://cplusplus.com/reference/chrono/system_clock/ below:

class

std::chrono::system_clock

System clock

Clock classes provide access to the current time_point.

Specifically, system_clock is a system-wide realtime clock.

Clock properties

realtime: It is intended to represent the real time, and thus it can be translated in some way to and from calendar representations (see to_time_t and from_time_t member functions).
signed count: Its time_point values can refer to times before the epoch (with negative values).
system-wide: All processes running on the system shall retrieve the same time_point values by using this clock.

Member types The following aliases are member types of system_clock:

member type definition notes rep A signed arithmetic type (or a class that emulates it) Used to store a count of periods. period A ratio type Represents the length of a period in seconds. duration duration<rep,period> The clock's duration type. time_point time_point<system_clock> The clock's time_point type.

Member constants member constant definition is_steady a bool value specifying whether the clock always advances, and whether it does at a steady state relative to physical time. If true, this implies that the system clock may not be adjusted.
Static member functions

now: Get current time (public static member function)

to_time_t: Convert to time_t (public static member function)

from_time_t: Convert from time_t (public static member function)

Example

// system_clock example
#include <iostream>
#include <ctime>
#include <ratio>
#include <chrono>

int main ()
{
  using std::chrono::system_clock;

  std::chrono::duration<int,std::ratio<60*60*24> > one_day (1);

  system_clock::time_point today = system_clock::now();
  system_clock::time_point tomorrow = today + one_day;

  std::time_t tt;

  tt = system_clock::to_time_t ( today );
  std::cout << "today is: " << ctime(&tt);

  tt = system_clock::to_time_t ( tomorrow );
  std::cout << "tomorrow will be: " << ctime(&tt);

  return 0;
}

Possible output:

today is: Wed May 30 12:25:03 2012
tomorrow will be: Thu May 31 12:25:03 2012