std::common_type(std::chrono::time_point)

template <class Clock, class Duration1, class Duration2>
struct common_type<std::chrono::time_point<Clock, Duration1>, 
                   std::chrono::time_point<Clock, Duration2>> {
    typedef std::chrono::time_point<
        Clock, typename std::common_type<Duration1, Duration2>::type> type;
};