title: Using C++20 chrono, how to compute various facts about a date
body: <p><a href="https://www.timeanddate.com/date/weekday.html" rel="nofollow noreferrer">https://www.timeanddate.com/date/weekday.html</a> computes various facts about a day of the year, for example:</p>
  
  <p><a href="https://i.stack.imgur.com/WPWuO.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/WPWuO.png" alt="https://i.stack.imgur.com/WPWuO.png"></a></p>
  
  <p>Given an arbitrary date, how can these numbers be computed with the <a href="http://eel.is/c++draft/#time" rel="nofollow noreferrer">C++20 chrono specification</a>?</p>