How do you extract specific date and time components, like year, month, day, hours, minutes, seconds, and milliseconds, from a `std::chrono::time_point` object in C ?

Extracting Date and Time Components from std::chrono::time_point

Problem:

How can we retrieve specific date and time components, such as year, month, day, hours, minutes, seconds, and milliseconds, from an std::chrono::time_point object?

Solution:

To extract this information, we must utilize the system_clock::time_point as it's the only system-supplied clock that has a connection to the civil calendar.

<code class="cpp">system_clock::time_point now = system_clock::now();</code>

Converting to a Time Stamp:

Next, convert the time_point to a time_t:

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

Extracting Components:

Using the C library, convert the time_t to a tm, specifying whether you want conversion in UTC or the local time zone:

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

Now, you can print the components of the tm:

<code class="cpp">std::cout <p><strong>Extended Extraction:</strong></p>
<p>While system_clock is typically based on Unix time, its precision is often greater than seconds. To access the high-resolution components, use the following code:</p>
<pre class="brush:php;toolbar:false"><code class="cpp">using namespace std;
using namespace std::chrono;

int main()
{
    typedef duration<int ratio_multiply ratio> >>::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;

    cout <p>By converting the time_point to days, hours, minutes, seconds, and extracting the fraction of a second, this code outputs:</p>
<pre class="brush:php;toolbar:false">15806d 20:31:14 598155[1/1000000]

Library Solutions:

The header-only date library simplifies this process, allowing the extraction with the following code:

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

int main()
{
    auto tp = std::chrono::system_clock::now();
    auto dp = date::floor<:days>(tp);
    auto ymd = date::year_month_day{dp};
    auto time = date::make_time(std::chrono::duration_cast<:chrono::milliseconds>(tp - dp));

    std::cout <p><strong>C  20 Integration:</strong></p>
<p>In C  20, a proposed syntax is included for extracting these fields using system_clock::time_point:</p>
<pre class="brush:php;toolbar:false"><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();
}</milliseconds></days></chrono></code>

Additionally, you can specify a preferred time zone for the extraction.

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