How can I extract individual temporal elements like year, month, day, hour, minute, second, and milliseconds from an std::chrono::time_point object?

Extracting Temporal Elements from an std::chrono::time_point

Query:

How can I extract specific temporal elements, such as year, month, day, hour, minute, second, and milliseconds, from an std::chrono::time_point object?

Resolution:

To extract these elements from a time_point, you must use the system_clock::time_point, as it has a relationship with the civil calendar. Retrieve the current system_clock::time_point:

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

Convert this to a time_t value:

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

If you require time extraction in the UTC timezone or local timezone, convert the time_t value to a tm:

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

Extract specific elements using the corresponding fields of the tm structure:

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

Expanded Information:

While using the C library allows for time conversion, it is important to note that every system_clock implementation is based on Unix time, typically measured in seconds since January 1, 1970 UTC.

For more precise extraction, take advantage of the system_clock precision:

<code class="cpp">system_clock::duration tp = now.time_since_epoch();
days d = duration_cast<days>(tp);  // Days
tp -= d;  // Subtract days
hours h = duration_cast<hours>(tp);  // Hours
tp -= h;  // Subtract hours
...  // Continue for minutes, seconds, and fractional seconds</code>

Print the results with the appropriate duration units:

<code class="cpp">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";

Library Enhancements:

For simplified time extraction, consider using a C 11/14 library such as date. This reduces the necessary code to:

<code class="cpp">auto tp = std::chrono::system_clock::now();
using date::floor;  // Namespace for streaming operator
std::cout << tp << '\n';  // Outputs date and time components
auto dp = floor<date::days>(tp);  // Omits fractional parts
auto ymd = date::year_month_day{dp};
...  // Individual elements can be accessed as members of ymd</code>

C 20 Standard Proposal:

The proposed C 20 working draft includes direct syntax for extracting these fields from system_clock::time_point:

<code class="cpp">year_month_day ymd{floor<days>(tp)};
hh_mm_ss time{floor<milliseconds>(tp - dp)};
auto y = ymd.year();
auto m = ymd.month();
...  // Extract remaining elements similarly</code>

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