std::chrono::operator<< (std::chrono::day)
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                    |   template <class CharT, class Traits> std::basic_ostream<CharT, Traits>&  | 
(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. Inserts that string into os. Then, if !d.ok(), inserts the string " is not a valid day" into os.
Return value
os.
Notes
This operator<< is primarily intended for debugging use. For control over formatting, use std::chrono::format.
See also
|    (C++20)  | 
  formats a streamable chrono object for insertion (function template)  | 
   outputs a day into a stream according to the provided format  (function template)  |