template <class Clock, class Duration1, class Rep2, class Period2>
time_point<Clock, typename common_type<Duration1, duration<Rep2, Period2>>::type>
operator+(const time_point<Clock, Duration1>& lhs, const duration<Rep2, Period2>& rhs);
Returns: CT(lhs) += rhs, where CT is the type of the return value.
template <class Rep1, class Period1, class Clock, class Duration2>
time_point<Clock, typename common_type<duration<Rep1, Period1>, Duration2>::type>
operator+(const duration<Rep1, Period1>& lhs, const time_point<Clock, Duration2>& rhs);
Returns: rhs + lhs.
template <class Clock, class Duration1, class Rep2, class Period2>
time_point<Clock, typename common_type<Duration1, duration<Rep2, Period2>>::type>
operator-(const time_point<Clock, Duration1>& lhs, const duration<Rep2, Period2>& rhs);
Returns: lhs + (-rhs).
template <class Clock, class Duration1, class Duration2>
typename common_type<Duration1, Duration2>::type
operator-(const time_point<Clock, Duration1>& lhs, const time_point<Clock, Duration2>& rhs);
Returns: lhs.time_since_epoch() - rhs.time_since_epoch().