Twhitney chrono (#3530)

* chrono c++20 work

* first draft

* draft

* fix links

* testing formatting

* add format options

* finish draft for chrono::days

* start month, couple fixes for day

* month draft

* tweak to spaceship operator description

* drafting year

* draft year

* draft weekday, update missing operators

* acrolinx

* finish year

* fix weekday sample

* add hh_mm_ss class docs

* add weekday-last-class

* added weekday_indexed-class, updated code snippets

* integrate Colin's work and update some code examples

* fix links, incorp tech review

* add flag/class table

* fix links

* fix filenames, links

* acrolinx

* tech review

* PR review feedback

* pr review feedback

* fix link

Co-authored-by: TylerMSFT <Tyler.Whitney@microsoft.com>

Author: TylerMSFT

docs/standard-library/chrono-functions.md

@@ -2,7 +2,7 @@
 description: "Learn more about: `chrono` literals"
 title: "chrono literals"
 f1_keywords: ["chrono/std::literals::chrono_literals", "std::literals::chrono_literals", "chrono_literals"]
-ms.date: "01/15/2021"
+ms.date: "4/23/2021"
 ---
 # `chrono` literals
 
@@ -13,6 +13,9 @@ ms.date: "01/15/2021"
 ```cpp
 inline namespace literals {
   inline namespace chrono_literals {
+    // return integral days
+    constexpr std::chrono::day operator ""d(unsigned long long d) noexcept;
+
     // return integral hours
     constexpr chrono::hours operator"" h(unsigned long long Val);
 
@@ -49,6 +52,8 @@ inline namespace literals {
     // return floating-point nanoseconds
     constexpr chrono::duration<double, nano> operator"" ns(long double Val);
 
+    // return integral year
+    constexpr chrono::year operator""y(unsigned long long y) noexcept; // C++ 20
   } // inline namespace chrono_literals
 } // inline namespace literals
 ```
@@ -62,7 +67,9 @@ The literals that take a **`long long`** argument return a value or the correspo
 The following examples show how to use the `chrono` literals.
 
 ```cpp
-constexpr auto day = 24h;
+constexpr auto day = 14d; // If the value > 255`, then the stored value is unspecified. 
+constexpr auto twoDays = 48h;
 constexpr auto week = 24h* 7;
 constexpr auto my_duration_unit = 108ms;
+constexpr auto theYear = 2021y;
 ```