How do you extract specific temporal units from a chrono::time_point in C ?

Extracting Temporal Units from Chrono Time Points

In C , extracting precise temporal units from an std::chrono::time_point object depends on the underlying clock type. The system_clock, which has a connection to the civil calendar, enables accessing data like year, month, day, and millisecond.

To obtain a system_clock::time_point representing the current time:

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

Convert it to a time_t and then to a tm struct using the C library. This conversion option can be either in UTC or the local time zone:

<code class="cpp">std::time_t tt = std::chrono::system_clock::to_time_t(now);
std::tm utc_tm = *std::gmtime(&tt);
std::tm local_tm = *std::localtime(&tt);</code>

You can then extract the desired temporal components, for example:

<code class="cpp">int year = local_tm.tm_year + 1900;
int month = local_tm.tm_mon + 1;
int day = local_tm.tm_mday;</code>

Additionally, system_clock is often based on Unix time, providing access to fractional seconds:

<code class="cpp">int frac_second = std::chrono::system_clock::duration::period::num / std::chrono::system_clock::duration::period::den;</code>

The duration_cast and custom duration class days can be utilized to facilitate precise time calculations:

<code class="cpp">std::chrono::duration<int, std::ratio_multiply<std::chrono::hours::period, std::ratio<24>>> days;
std::chrono::days d = std::chrono::duration_cast<std::chrono::days>(tp);
tp -= d;
std::chrono::hours h = std::chrono::duration_cast<std::chrono::hours>(tp);
// ... (continue for minutes, seconds, and fractional seconds)</code>

Alternatively, the "date" library offers a simplified and versatile approach:

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

auto tp = std::chrono::system_clock::now();
auto dp = date::floor<date::days>(tp);
auto ymd = date::year_month_day{dp};
auto time = date::make_time(std::chrono::duration_cast<std::chrono::milliseconds>(tp-dp));
int year = ymd.year();
int month = ymd.month();
// ... (continue for day, hour, minute, second, millisecond)</code>

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