[time.point.nonmember]

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template<class Clock, class Duration1, class Rep2, class Period2>
  constexpr time_point<Clock, common_type_t<Duration1, duration<Rep2, Period2>>
    operator+(const time_point<Clock, Duration1>& lhs, const duration<Rep2, Period2>& rhs);

Returns: CT(lhs.time_since_epoch() + rhs), where CT is the type of the return value.

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template<class Rep1, class Period1, class Clock, class Duration2>
  constexpr time_point<Clock, common_type_t<duration<Rep1, Period1>, Duration2>>
    operator+(const duration<Rep1, Period1>& lhs, const time_point<Clock, Duration2>& rhs);

Returns: rhs + lhs.

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template<class Clock, class Duration1, class Rep2, class Period2>
  constexpr time_point<Clock, common_type_t<Duration1, duration<Rep2, Period2>>
    operator-(const time_point<Clock, Duration1>& lhs, const duration<Rep2, Period2>& rhs);

Returns: CT(lhs.time_since_epoch() - rhs), where CT is the type of the return value.

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template<class Clock, class Duration1, class Duration2>
  constexpr common_type_t<Duration1, Duration2>
    operator-(const time_point<Clock, Duration1>& lhs, const time_point<Clock, Duration2>& rhs);

Returns: lhs.time_since_epoch() - rhs.time_since_epoch().

30 Time library [time]

30.6 Class template time_point [time.point]

30.6.6 Non-member arithmetic [time.point.nonmember]