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.


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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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