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

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

Synopsis

constexpr bool ok() const noexcept; (since C++20)
Checks if the day value stored in *this is in the valid range, i.e., [1, 31].

Return value

true if the day value stored in *this is in the range [1, 31]. Otherwise false.