template <class ToDuration, class Clock, class Duration>
constexpr time_point<Clock, ToDuration>
time_point_cast(const time_point<Clock, Duration>& t);
template <class ToDuration, class Clock, class Duration>
constexpr time_point<Clock, ToDuration>
floor(const time_point<Clock, Duration>& tp);
template <class ToDuration, class Clock, class Duration>
constexpr time_point<Clock, ToDuration>
ceil(const time_point<Clock, Duration>& tp);
template <class ToDuration, class Clock, class Duration>
constexpr time_point<Clock, ToDuration>
round(const time_point<Clock, Duration>& tp);