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

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< cpp‎ | chrono‎ | day

Defined in header `<chrono>` |  |
template< class CharT, class Traits > std::basic_ostream<CharT, Traits>& operator<<( std::basic_ostream<CharT, Traits>& os, const std::chrono::day& d ); |  | (since C++20)

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

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