std::chrono::weekday::c_encoding, std::chrono::weekday::iso_encoding
From cppreference.com
                    
                                        
                    
                    
                                                            
                    | constexpr unsigned c_encoding() const noexcept; | (1) | (since C++20) | 
| constexpr unsigned iso_encoding() const noexcept; | (2) | (since C++20) | 
1) Returns the weekday value stored in 
*this.1) Returns the weekday value stored in 
*this, except that Sunday (stored as 0) is interpreted as 7 according to ISO 8601.Return value
1) The weekday value stored in 
*this.2) c_encoding() == 0u ? 7u : c_encoding().