C++20: Query Calendar Dates and Ordinal Dates

// queryCalendarDates.cpp

#include <chrono>
#include <iostream>

int main() {

    std::cout << '\n';

    using std::chrono::floor;

    using std::chrono::January;

    using std::chrono::years;
    using std::chrono::days;
    using std::chrono::hours;

    using std::chrono::year_month_day;
    using std::chrono::year_month_weekday;

    using std::chrono::sys_days;

    auto now = std::chrono::system_clock::now();              
    std::cout << "The current time is: " << now << " UTC\n";                     // (1)   
    std::cout << "The current date is: " << floor<days>(now) << '\n';
    std::cout << "The current date is: " << year_month_day{floor<days>(now)} 
              << '\n';
    std::cout << "The current date is: " << year_month_weekday{floor<days>(now)} 
              << '\n';

    std::cout << '\n';

    
    auto currentDate = year_month_day(floor<days>(now));                         // (2)
    auto currentYear = currentDate.year();
    std::cout << "The current year is " << currentYear << '\n';    
    auto currentMonth = currentDate.month();
    std::cout << "The current month is " << currentMonth << '\n'; 
    auto currentDay = currentDate.day();
    std::cout << "The current day is " << currentDay << '\n'; 

    std::cout << '\n';
                                                               
    auto hAfter = floor<hours>(now) - sys_days(January/1/currentYear);           // (3)
    std::cout << "It has been " << hAfter << " since New Year!\n";  
    auto nextYear = currentDate.year() + years(1);             
    auto nextNewYear = sys_days(January/1/nextYear);
    auto hBefore =  sys_days(January/1/nextYear) - floor<hours>(now);            // (4)
    std::cout << "It is " << hBefore << " before New Year!\n";

    std::cout << '\n';
                                                            
    std::cout << "It has been " << floor<days>(hAfter) << " since New Year!\n";  // (5)
    std::cout << "It is " << floor<days>(hBefore) << " before New Year!\n";      // (6)
    
    std::cout << '\n';
    
}

// weekdaysOfBirthdays.cpp

#include <chrono>
#include <cstdlib>
#include <iostream>

int main() {

    std::cout << '\n';

    int y;
    int m;
    int d;

    std::cout << "Year: ";                                            // (1)                          
    std::cin >> y;
    std::cout << "Month: ";
    std::cin >> m;
    std::cout << "Day: ";
    std::cin >> d;

    std::cout << '\n';

    auto birthday = std::chrono::year(y)/std::chrono::month(m)/std::chrono::day(d);  // (2)      

    if (not birthday.ok()) {                                         // (3)                     
        std::cout << birthday << '\n';
        std::exit(EXIT_FAILURE);
    }

    std::cout << "Birthday: " << birthday << '\n';
    auto birthdayWeekday = std::chrono::year_month_weekday(birthday); // (4)  
    std::cout << "Weekday of birthday: " << birthdayWeekday.weekday() << '\n';

    auto currentDate = std::chrono::year_month_day(
        std::chrono::floor<std::chrono::days>(std::chrono::system_clock::now()));  
    auto currentYear = currentDate.year();
    
    auto age = static_cast<int>(currentDate.year()) - 
               static_cast<int>(birthday.year());                     // (5)
    std::cout << "Your age: " << age << '\n';

    std::cout << '\n';

    std::cout << "Weekdays for your next 10 birthdays" << '\n';  

    for (int i = 1, newYear = (int)currentYear; i <= 10;  ++i ) {    // (6)
        std::cout << "  Age " <<  ++age << '\n';
        auto newBirthday = std::chrono::year(++newYear)/
                           std::chrono::month(m)/std::chrono::day(d);
        std::cout << "    Birthday: " << newBirthday << '\n';
        std::cout << "    Weekday of birthday: " 
                  << std::chrono::year_month_weekday(newBirthday).weekday() << '\n';
    }

    std::cout << '\n';

}

// ordinalDate.cpp

#include "date.h"
#include <iomanip>
#include <iostream>

int main()
{
   using namespace date;
   
   const auto time = std::chrono::system_clock::now();
   const auto daypoint = floor<days>(time);                    // (1) 
   const auto ymd = year_month_day{daypoint};         
   
   // calculating the year and the day of the year
   const auto year = ymd.year();
   const auto year_day = daypoint - sys_days{year/January/0}; // (2) 
                                                              // (3)
   std::cout << year << '-' << std::setfill('0') << std::setw(3) << year_day.count() << std::endl;
   
   // inverse calculation and check
   assert(ymd == year_month_day{sys_days{year/January/0} + year_day});
}