C++ | Date and time using <chrono> header

Measuring time using

The system_clock can be used to measure the time elapsed during some part of a program’s execution.

#include <iostream>
#include <chrono>
#include <thread>

int main() {
    auto start = std::chrono::system_clock::now(); // This and "end"'s type is std::chrono::time_point
    { // The code to test
        std::this_thread::sleep_for(std::chrono::seconds(2));
    }
    auto end = std::chrono::system_clock::now();

    std::chrono::duration<double> elapsed = end - start;
    std::cout << "Elapsed time: " << elapsed.count() << "s";
}

In this example, sleep_for was used to make the active thread sleep for a time period measured in std::chrono::seconds, but the code between braces could be any function call that takes some time to execute.

Find number of days between two dates

This example shows how to find number of days between two dates. A date is specified by year/month/day of month, and additionally hour/minute/second.

Program calculates number of days in years since 2000.

#include <iostream>
#include <string>
#include <chrono>
#include <ctime>

/***
 * Creates a std::tm structure from raw date.
 *
 * \param year (must be 1900 or greater)
 * \param month months since January – [1, 12]
 * \param day day of the month – [1, 31]
 * \param minutes minutes after the hour – [0, 59]
 * \param seconds seconds after the minute – [0, 61](until C++11) / [0, 60] (since C++11)
 *
 * Based on http://en.cppreference.com/w/cpp/chrono/c/tm
 */
std::tm CreateTmStruct(int year, int month, int day, int hour, int minutes, int seconds) {
    struct tm tm_ret = {0};

    tm_ret.tm_sec = seconds;
    tm_ret.tm_min = minutes;
    tm_ret.tm_hour = hour;
    tm_ret.tm_mday = day;
    tm_ret.tm_mon = month - 1;
    tm_ret.tm_year = year - 1900;

    return tm_ret;
}

int get_days_in_year(int year) {

    using namespace std;
    using namespace std::chrono;

    // We want results to be in days
    typedef duration<int, ratio_multiply<hours::period, ratio<24> >::type> days;

    // Create start time span
    std::tm tm_start = CreateTmStruct(year, 1, 1, 0, 0, 0);
    auto tms = system_clock::from_time_t(std::mktime(&tm_start));

    // Create end time span
        std::tm tm_end =   CreateTmStruct(year + 1, 1, 1, 0, 0, 0);
    auto tme = system_clock::from_time_t(std::mktime(&tm_end));

    // Calculate time duration between those two dates
    auto diff_in_days = std::chrono::duration_cast<days>(tme - tms);

    return diff_in_days.count();
}

int main()
{
    for ( int year = 2000; year <= 2016; ++year )
        std::cout << "There are " << get_days_in_year(year) << " days in " << year << "\n";
}