namespace std::chrono { class file_clock { public: using rep = a signed arithmetic type; using period = ratio<unspecified, unspecified>; using duration = chrono::duration<rep, period>; using time_point = chrono::time_point<file_clock>; static constexpr bool is_steady = unspecified; static time_point now() noexcept; // Conversion functions, see below }; }