std::chrono::operator<<(std::chrono::day)(3)

NAME
std::chrono::operator<<(std::chrono::day) - std::chrono::operator<<(std::chrono::day)

Synopsis
   Defined in header <chrono>
   template< class CharT, class Traits >
   std::basic_ostream<CharT, Traits>& (since C++20)
       operator<<( std::basic_ostream<CharT, Traits>& os, const
   std::chrono::day& d );
   Forms a std::basic_string<CharT> s consisting of the day value stored in d formatted
   as a decimal number, with a leading zero if the result would otherwise be a single
   decimal digit. Then, if !d.ok(), append " is not a valid day" to the formatted
   string. Inserts that string into os.
   Equivalent to
   return os << (d.ok() ?
       std::format(STATICALLY_WIDEN<CharT>("{:%d}"), d) :
       std::format(STATICALLY_WIDEN<CharT>("{:%d} is not a valid day"), d));
   where STATICALLY_WIDEN<CharT>("...") is "..." if CharT is char, and L"..." if CharT
   is wchar_t.

Return value
   os

Example
// Run this code
 #include <chrono>
 #include <iostream>
 int main()
 {
     constexpr std::chrono::day d1{31}, d2{7}, d3{42}, d4{};
     std::cout << d1 << '\n'
               << d2 << '\n'
               << d3 << '\n'
               << d4 << '\n';
 }

Possible output:
 31
 07
 42 is not a valid day
 00 is not a valid day

See also
   format stores formatted representation of the arguments in
   (C++20) a new string
                        (function template)
   std::formatter<std::chrono::day> formatting support for day
   (C++20) (class template specialization)