std::chrono::day::day (3) Linux Manual Page

std::chrono::day::day – std::chrono::day::day

Synopsis

day() = default;                             (1) (since C++20)

explicit constexpr 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.