std::chrono::month_day::month_day
From cppreference.com
                    
                                        
                    
                    
                                                            
                    |   month_day() = default;  | 
(1) | (since C++20) | 
|   constexpr month_day(const std::chrono::month& m, const std::chrono::day& d) noexcept;  | 
(2) | (since C++20) | 
Constructs a month_day.
1) Default constructor leaves the stored month and day values uninitialized.
2) Constructs a 
month_day that stores the month m and the day d.Notes
A more convenient way to construct a month_day is with operator/, e.g., std::chrono::April/1. 
See also
|    (C++20)  | 
   conventional syntax for Gregorian calendar date creation   (function)  |