C++20: Displaying and Checking Calendar Dates

I created calendar dates in my last post, “C++20: Creating Calendar Dates,” and I will display and check them today.

This post is the fifth in my detailed journey through the chrono extension in C++20:

Displaying Calendar Dates

Thanks to std::chrono::local_days or std::chrono::sys_days, you can convert calendar dates to a local or a system std::chrono::time_point. I use std::chrono::sys_days in my example. std::chrono::sys_days is based on std::chrono::system_clock.

// sysDays.cpp

#include <chrono>
#include <iostream>
 
int main() {

    std::cout << '\n';

    using std::chrono::last;

    using std::chrono::year;
    using std::chrono::sys_days;

    using std::chrono::March;
    using std::chrono::February;

    using std::chrono::Monday;
    using std::chrono::Thursday;

    constexpr auto yearMonthDayLast{year(2010)/March/last};               // (1)
    std::cout << "sys_days(yearMonthDayLast): " 
              << sys_days(yearMonthDayLast)  <<  '\n';

    constexpr auto yearMonthWeekday{year(2020)/March/Thursday[2]};
    std::cout << "sys_days(yearMonthWeekday): " 
              <<  sys_days(yearMonthWeekday) << '\n';

    constexpr auto yearMonthWeekdayLast{year(2010)/March/Monday[last]};
    std::cout << "sys_days(yearMonthWeekdayLast): " 
              << sys_days(yearMonthWeekdayLast) << '\n';

    std::cout << '\n';

    constexpr auto leapDate{year(2012)/February/last};               
    std::cout << "sys_days(leapDate): " << sys_days(leapDate) << '\n';    // (2)

    constexpr auto noLeapDate{year(2013)/February/last};             
    std::cout << "sys_day(noLeapDate): " << sys_days(noLeapDate) << '\n'; // (3)

    std::cout << '\n';

}

The std::chrono::last constant (line 1) lets me quickly determine how many days a month has. The output shows that 2012 is a leap year (line 2), but not 2013 (line 3).

Suppose you have a calendar date like year(2100)/2/29. Your first question may be: Is this date valid?

Check if a Date is Valid

The various calendar types in C++20 have the function ok. This function returns true if the date is valid.

//  leapYear.cpp

#include <chrono>
#include <iostream>
 
int main() {

    std::cout << std::boolalpha << '\n';

    std::cout << "Valid days" << '\n';                         // (1)
    std::chrono::day day31(31);
    std::chrono::day day32 = day31 + std::chrono::days(1);
    std::cout << "  day31: " << day31 << "; ";
    std::cout << "day31.ok(): "  << day31.ok() << '\n';
    std::cout << "  day32: " << day32 << "; ";
    std::cout << "day32.ok(): "  << day32.ok() << '\n';
   

    std::cout << '\n';

    std::cout << "Valid months" << '\n';                      // (2)
    std::chrono::month month1(1);
    std::chrono::month month0(0);
    std::cout << "  month1: " << month1 << "; ";
    std::cout << "month1.ok(): "  << month1.ok() << '\n';
    std::cout << "  month0: " << month0 << "; ";
    std::cout << "month0.ok(): "  << month0.ok() << '\n';

    std::cout << '\n';

    std::cout << "Valid years" << '\n';                       // (3)
    std::chrono::year year2020(2020);
    std::chrono::year year32768(-32768);
    std::cout << "  year2020: " << year2020 << "; ";
    std::cout << "year2020.ok(): "  << year2020.ok() << '\n';
    std::cout << "  year32768: " << year32768 << "; ";
    std::cout << "year32768.ok(): "  << year32768.ok() << '\n';

    std::cout << '\n';

    std::cout << "Leap Years"  << '\n';       

    constexpr auto leapYear2016{std::chrono::year(2016)/2/29};
    constexpr auto leapYear2020{std::chrono::year(2020)/2/29};
    constexpr auto leapYear2024{std::chrono::year(2024)/2/29};

    std::cout << "  leapYear2016.ok(): " << leapYear2016.ok() << '\n';
    std::cout << "  leapYear2020.ok(): " << leapYear2020.ok() << '\n';
    std::cout << "  leapYear2024.ok(): " << leapYear2024.ok() << '\n';

    std::cout << '\n';

     std::cout << "No Leap Years"  << '\n';   

    constexpr auto leapYear2100{std::chrono::year(2100)/2/29};
    constexpr auto leapYear2200{std::chrono::year(2200)/2/29};
    constexpr auto leapYear2300{std::chrono::year(2300)/2/29};

    std::cout << "  leapYear2100.ok(): " << leapYear2100.ok() << '\n';
    std::cout << "  leapYear2200.ok(): " << leapYear2200.ok() << '\n';
    std::cout << "  leapYear2300.ok(): " << leapYear2300.ok() << '\n';

    std::cout << '\n';

    std::cout << "Leap Years"  << '\n';      

    constexpr auto leapYear2000{std::chrono::year(2000)/2/29};
    constexpr auto leapYear2400{std::chrono::year(2400)/2/29};
    constexpr auto leapYear2800{std::chrono::year(2800)/2/29};

    std::cout << "  leapYear2000.ok(): " << leapYear2000.ok() << '\n';
    std::cout << "  leapYear2400.ok(): " << leapYear2400.ok() << '\n';
    std::cout << "  leapYear2800.ok(): " << leapYear2800.ok() << '\n';

    std::cout << '\n';

}

I check in the program if a given day (line 1), a given month (line 2), or a given year (line 3) is valid. The range of a day is [1, 31], of a month [1, 12], and a year [ -32767, 32767]. Consequently, the ok() calls on the corresponding values return false. Two facts are interesting when I display various values. First, if the value is not valid, the output shows: “is not a valid day”, “is not a valid month”, “is not a valid year”. Second, the values are displayed in string representation.

You can apply the ok-call on a calendar date. Now it’s pretty easy to check if a specific calendar date is a leap day and, therefore, the corresponding year a leap year. In the worldwide used Gregorian calendar, the following rules apply:

Each year that is exactly divisible by 4 is a leap year.

Except for years that are exactly divisible by 100. They are not leap years.
Except for years that are exactly divisible by 400. They are leap years.

Too complicated? The program leapYears.cpp exemplifies this rule.

Modernes C++ Mentoring

"Fundamentals for C++ Professionals" (open)

"Design Patterns and Architectural Patterns with C++" (open)

"C++20: Get the Details" (open)

"Concurrency with Modern C++" (open)

"Generic Programming (Templates) with C++": October 2024

"Embedded Programming with Modern C++": October 2024

"Clean Code: Best Practices for Modern C++": March 2025

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What’s Next?

The extended chrono library makes it relatively easy to ask for the time duration between calendar dates.

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