Extracting Temporal Components from std::chrono::time_point

Objective: Acquiring the year, month, day, hour, minute, second, and milliseconds from an std::chrono::time_point object.

Solution:

To extract this information, you specifically require a system_clock::time_point, providing a relationship to the civil calendar system.

<code class="cpp">using namespace std::chrono;
system_clock::time_point now = system_clock::now();</code>

The next step is converting it to a time_t using:

<code class="cpp">time_t tt = system_clock::to_time_t(now);</code>

Utilize the C library to convert time_t to tm, ensuring the right time zone is chosen:

<code class="cpp">tm utc_tm = *gmtime(&amp;tt);
tm local_tm = *localtime(&amp;tt);</code>

Now, you can extract the components of tm:

<code class="cpp">std::cout << local_tm.tm_year + 1900 << '\n';
std::cout << local_tm.tm_mon + 1 << '\n';
std::cout << local_tm.tm_mday << '\n';

Extracting System Clock Components

In addition, if you prefer, you can rely on the following non-guaranteed approach:

Every implementation of system_clock known to most is based on Unix time (the number of seconds since January 1, 1970 UTC), usually with a finer precision than seconds.

The following comprehensive program provides an example of extracting all temporal components:

<code class="cpp">#include <chrono>
#include <ctime>
#include <iostream>

int main() {
  using namespace std::chrono;
  typedef duration<int, ratio_multiply<hours::period, ratio<24> >::type> days;
  system_clock::time_point now = system_clock::now();
  system_clock::duration tp = now.time_since_epoch();
  days d = duration_cast<days>(tp);
  tp -= d;
  hours h = duration_cast<hours>(tp);
  tp -= h;
  minutes m = duration_cast<minutes>(tp);
  tp -= m;
  seconds s = duration_cast<seconds>(tp);
  tp -= s;
  std::cout << d.count() << "d " << h.count() << ':' << m.count() << ':' << s.count();
  std::cout << " " << tp.count() << "[" << system_clock::duration::period::num << '/' << system_clock::duration::period::den << "]\n";

  time_t tt = system_clock::to_time_t(now);
  tm utc_tm = *gmtime(&amp;tt);
  tm local_tm = *localtime(&amp;tt);
  std::cout << utc_tm.tm_year + 1900 <&lt '-';
  std::cout << utc_tm.tm_mon + 1 << '-';
  std::cout << utc_tm.tm_mday << ' ';
  std::cout << utc_tm.tm_hour << ':';
  std::cout << utc_tm.tm_min << ':';
  std::cout << utc_tm.tm_sec << '\n';
}</code>

Using a Custom Duration

To model days, create a custom duration:

<code class="cpp">typedef duration<int, ratio_multiply<hours::period, ratio<24> >::type> days;</code>

Now you can get the time since the epoch with the system_clock::duration's precision:

<code class="cpp">system_clock::duration tp = now.time_since_epoch();</code>

Truncate it to days and subtract that:

<code class="cpp">days d = duration_cast<days>(tp);
tp -= d;</code>

Repeat this process for hours, minutes, and seconds.

The remainder is the fraction of a second in the units of system_clock::duration.

Using a Date Library

This C 11/14 library streamlines the above process:

<code class="cpp">#include "date.h"
#include <iostream>

int main() {
  using C = std::chrono;
  using D = date;
  using S = std;

  auto tp = C::system_clock::now();
  {
    using namespace date;
    S::cout << tp << '\n';
  }
  auto dp = D::floor<D::days>(tp);
  auto ymd = D::year_month_day{dp};
  auto time = D::make_time(C::duration_cast<C::milliseconds>(tp - dp));
  S::cout << "year        = " << ymd.year() << '\n';
  S::cout << "month       = " << ymd.month() << '\n';
  S::cout << "day         = " << ymd.day() << '\n';
  S::cout << "hour        = " << time.hours().count() << "h\n";
  S::cout << "minute      = " << time.minutes().count() << "min\n";
  S::cout << "second      = " << time.seconds().count() << "s\n";
  S::cout << "millisecond = " << time.subseconds().count() << "ms\n";
}

C 20 Standard Library Support

C 20 introduces a standardized syntax for extracting these fields from a system_clock::time_point:

<code class="cpp">#include <chrono>

int main() {
  using namespace std::chrono;
  auto tp = system_clock::now();
  auto dp = floor<days>(tp);
  year_month_day ymd{dp};
  hh_mm_ss time{floor<milliseconds>(tp - dp)};
  auto y = ymd.year();
  auto m = ymd.month();
  auto d = ymd.day();
  auto h = time.hours();
  auto M = time.minutes();
  auto s = time.seconds();
  auto ms = time.subseconds();
}</code>

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