std::chrono::day::day
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                    |   day() = default;  | 
(1) | (since C++20) | 
|   constexpr explicit day( unsigned d ) noexcept;  | 
(2) | (since C++20) | 
Constructs a day object.
1) Default constructor leaves the day value uninitialized.
2) If d <= 255, constructs a 
day object holding the day value d. Otherwise the value held is unspecified.