C++20: Creating Calendar Dates

In this post, I will dive deeper into calendar dates and create them.

If this post is too overwhelming for you, read my previous ones:

C++20 supports constants and literals to make using calendar-date types more convenient.

Constants and Literals for Calendar Types

Let me start with the constants for std::chrono::weekday, and std::chrono::month.

std::chrono::weekday

std::chrono::Monday
std::chrono::Thuesday
std::chrono::Wednesday
std::chrono::Thursday
std::chrono::Friday
std::chrono::Saturday
std::chrono::Sunday

std::chrono::month

std::chrono::January
std::chrono::February
std::chrono::March
std::chrono::April
std::chrono::May
std::chrono::June
std::chrono::July
std::chrono::August
std::chrono::September
std::chrono::October
std::chrono::November
std::chrono::December

C++20 supports for calendar types std::chrono::day and std::chrono::year two new literals: d and y. You can read more details about it in the post Basic Chrono Terminology with Time Duration and Time Point.

Let me create a few calendar dates.

Create Calendar Dates

The program createCalendar.cpp shows various ways to create calendar-related dates.

// createCalendar.cpp

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

    std::cout << '\n';

    using namespace std::chrono_literals; 

    using std::chrono::last;

    using std::chrono::year;
    using std::chrono::month;
    using std::chrono::day;

    using std::chrono::year_month;
    using std::chrono::year_month_day;
    using std::chrono::year_month_day_last;
    using std::chrono::year_month_weekday;
    using std::chrono::year_month_weekday_last;
    using std::chrono::month_weekday;
    using std::chrono::month_weekday_last;
    using std::chrono::month_day;
    using std::chrono::month_day_last;
    using std::chrono::weekday_last;
    using std::chrono::weekday;

    using std::chrono::January;
    using std::chrono::February;
    using std::chrono::June;
    using std::chrono::March;
    using std::chrono::October;

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

    constexpr auto yearMonthDay{year(1940)/month(6)/day(26)};                // (1)
    std::cout << yearMonthDay << " ";
    std::cout << year_month_day(1940y, June, 26d) << '\n';                   // (2)

    std::cout << '\n';

    constexpr auto yearMonthDayLast{year(2010)/March/last};                  // (3)
    std::cout << yearMonthDayLast << " ";
    std::cout << year_month_day_last(2010y, month_day_last(month(3))) << '\n';

    constexpr auto yearMonthWeekday{year(2020)/March/Thursday[2]};            // (4)
    std::cout << yearMonthWeekday << " ";
    std::cout << year_month_weekday(2020y, month(March), Thursday[2]) << '\n';

    constexpr auto yearMonthWeekdayLast{year(2010)/March/Monday[last]};        //(5)
    std::cout << yearMonthWeekdayLast << " ";
    std::cout << year_month_weekday_last(2010y, month(March), weekday_last(Monday));

    std::cout << "\n\n";

    constexpr auto day_{day(19)};     // (6)         
    std::cout << day_  << " ";
    std::cout << day(19) << '\n';

    constexpr auto month_{month(1)};  // (7)     
    std::cout << month_  << " ";
    std::cout << month(1) << '\n';

    constexpr auto year_{year(1988)}; // (8)    
    std::cout << year_  << " ";
    std::cout << year(1988) << '\n';

    constexpr auto weekday_{weekday(5)};
    std::cout << weekday_  << " ";
    std::cout << weekday(5) << '\n';
 
    constexpr auto yearMonth{year(1988)/1};
    std::cout << yearMonth  << " ";
    std::cout << year_month(year(1988), January) << '\n';
 
    constexpr auto monthDay{10/day(22)};
    std::cout << monthDay <<  " ";
    std::cout << month_day(October, day(22)) << '\n';

    constexpr auto monthDayLast{June/last};
    std::cout << monthDayLast << " ";
    std::cout << month_day_last(month(6)) << '\n';
 
    constexpr auto monthWeekday{2/Monday[3]};
    std::cout << monthWeekday << " ";
    std::cout << month_weekday(February, Monday[3]) << '\n';
 
    constexpr auto monthWeekDayLast{June/Sunday[last]};
    std::cout << monthWeekDayLast << " ";
    std::cout << month_weekday_last(June, weekday_last(Sunday)) << '\n';

    std::cout << '\n';

}

There are two ways to create a calendar date. You can use the so-called cute syntax yearMonthDay{year(1940)/month(6)/day(26)} (line 1), or you can use the explicit type date::year_month_day(1940y, June, 26d) (line 2). To avoid overwhelming you, I will delay my explanation of the cute syntax to the next section. The explicit type is interesting because it uses the date-time literals 1940y, 26d, and the predefined constant June. This was the obvious part of the program.

Line 3, line 4, and line 5 offer additional ways to create calendar dates.

Line 3: the last day of March 2010: {year(2010)/March/last} or year_month_day_last(2010y,month_day_last(month(3)))
Line 4: the second Thursday of March 2020: {year(2020)/March/Thursday[2]} or year_month_weekday(2020y, month(March), Thursday[2])
Line 5: the last Monday of March 2010: {year(2010)/March/Monday[last]} or year_month_weekday_last(2010y, month(March), weekday_last(Monday))

The remaining calendar types stand for a day (line 6), a month (line 7), or a year (line 8). You can combine them as basic building blocks for fully specified calendar dates, such as in lines 3, 4, or 5.

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This is the output of the program:

As promised, let me write about the cute syntax.

Cute Syntax

The cute syntax consists of overloaded division operators to specify a calendar date. The overloaded operators support time literals (e.g., 2020y, 31d) and std::chrono::month constants such as std::chrono::January, std::chrono::February, …, std::chrono::December.

The following three combinations of year, month, and day are possible using the cute syntax.

year/month/day
day/month/year
month/day/year

These combinations are not chosen arbitrarily. They are the ones used most worldwide, and no other combination is allowed.

Consequently, when you choose the type year, month, or day for the first argument, the type for the remaining two arguments is no longer necessary, and a number does the job.

// cuteSyntax.cpp

#include <chrono>
#include <iostream>

int main() {

    std::cout << '\n';

    constexpr auto yearMonthDay{std::chrono::year(1966)/6/26};
    std::cout << yearMonthDay << '\n';

    constexpr auto dayMonthYear{std::chrono::day(26)/6/1966};
    std::cout << dayMonthYear << '\n';

    constexpr auto monthDayYear{std::chrono::month(6)/26/1966};
    std::cout << monthDayYear << '\n';

    constexpr auto yearDayMonth{std::chrono::year(1966)/std::chrono::month(26)/6};  
    std::cout << yearDayMonth << '\n';

    std::cout << '\n';

The last values for year/day/month are not allowed and cause a run-time message.

What’s Next

I assume you want to display a calendar date {year(2010)/March/last} in a readable form, for example, 2020-03-31. This is a job for the local_days or sys_days operator.

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