std::chrono::operator<<(std::chrono::month_day)
From cppreference.com
                    
                                        
                    
                    
                                                            
                    | template< class CharT, class Traits > std::basic_ostream<CharT, Traits>& | (since C++20) | |
Outputs a textual representation of md into the stream os, as if by
os << std::format(os.getloc(), STATICALLY_WIDEN<CharT>("{}/{}"), md.month(), md.day())
where STATICALLY_WIDEN<CharT>("...") is "..." if CharT is char, and L"..." if CharT is wchar_t.
Return value
os.
See also
| (C++20) | stores formatted representation of the arguments in a new string (function template) | 
| specialization of std::formatter that formats a month_dayaccording to the provided format(class template specialization) |