IntermediatePractice in the editor

C++ Date and Time (chrono)

The header chrono measures and handles time in a type-safe way: a duration of 5 seconds and one of 5 milliseconds are different types, so they cannot be mixed up by accident.

Durations

ExampleTry it in the editor

#include <chrono>
#include <iostream>

int main() {
    using namespace std::chrono;
    seconds s(90);
    minutes m = duration_cast<minutes>(s);
    milliseconds ms = s;                       // exact: converts automatically
    std::cout << s.count() << " s = " << m.count() << " min (rounded down) = "
              << ms.count() << " ms\n";
    auto total = hours(1) + minutes(30);
    std::cout << duration_cast<minutes>(total).count() << " minutes\n";
    return 0;
}

Output

90 s = 1 min (rounded down) = 90000 ms
90 minutes

Conversions that cannot lose information (seconds to milliseconds) happen automatically; conversions that can (seconds to minutes) need duration_cast, which cuts off the remainder.

Measuring how long code takes

steady_clock never jumps (unlike the wall clock, which can be changed), so it is the right clock for measuring:

ExampleTry it in the editor

#include <chrono>
#include <iostream>

int main() {
    auto start = std::chrono::steady_clock::now();
    volatile long long sum = 0;
    for (long long i = 0; i < 10000000; i++) sum += i;
    auto end = std::chrono::steady_clock::now();
    auto ms = std::chrono::duration_cast<std::chrono::milliseconds>(end - start);
    std::cout << "sum " << sum << " took " << ms.count() << " ms\n";
    return 0;
}

The time differs on every run and every computer, so this example has no fixed output.

The current date and time

system_clock::now() gives the current time. C++20 can print it directly with std::format, or you can convert it to C's time_t and use std::put_time:

ExampleTry it in the editor

#include <chrono>
#include <ctime>
#include <iomanip>
#include <iostream>

int main() {
    auto now = std::chrono::system_clock::now();
    std::time_t t = std::chrono::system_clock::to_time_t(now);
    std::cout << "Now: " << std::put_time(std::localtime(&t), "%Y-%m-%d %H:%M:%S") << "\n";
    return 0;
}

Calendar dates (C++20)

C++20 adds calendar types: years, months, days and weekdays, with checks for valid dates.

ExampleTry it in the editor

#include <chrono>
#include <iostream>

int main() {
    using namespace std::chrono;
    year_month_day d{year{2024}, month{1}, day{26}};
    weekday w{sys_days{d}};
    std::cout << "valid: " << std::boolalpha << d.ok() << ", weekday number " << w.c_encoding() << "\n";
    year_month_day bad{year{2023}, month{2}, day{29}};
    std::cout << "2023-02-29 valid: " << bad.ok() << "\n";
    auto later = sys_days{d} + days{10};
    std::cout << "10 days later: " << year_month_day{later}.day() << "\n";
    return 0;
}

Output

valid: true, weekday number 5
2023-02-29 valid: false
10 days later: 05

Weekday number 5 is Friday (Sunday is 0), and 10 days after 26 January is 5 February.

Good practice

Keep time values in chrono types for as long as possible and call count() only when printing; a function that takes std::chrono::milliseconds cannot be called with seconds by mistake. Use steady_clock to measure, system_clock for the date and time shown to people. To pause a program, std::this_thread::sleep_for(std::chrono::milliseconds(500)) from the thread header takes a duration directly.

Practice exercises

Convert 3725 seconds into hours, minutes and seconds with duration_cast.
Measure how long it takes to sort a vector of a million random numbers.
Check whether 29 February is a valid date in 2024, 2100 and 2000.