std::chrono::year_month::year_month
From cppreference.com
                    
                                        
                    < cpp | chrono | year month
                    
                                                            
                    |   year_month() = default;  | 
(1) | (since C++20) | 
|   constexpr year_month(const std::chrono::year& y, const std::chrono::month& m) noexcept;  | 
(2) | (since C++20) | 
Constructs a year_month object.
1) Default constructor leaves the year and month uninitialized.
2) Constructs a 
year_month object storing the year y and the month m.Notes
A more convenient way to construct a year_month is with operator/, e.g., 2007y/std::chrono::April.
See also
|    (C++20)  | 
   conventional syntax for Gregorian calendar date creation   (function)  |