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

std::chrono::month_weekday_last::ok – std::chrono::month_weekday_last::ok

Synopsis

constexpr bool ok() const noexcept; (since C++20)
Checks if the contained month and weekday_last objects are valid.

Return value

month().ok() && weekday_last().ok()