template<class Rep1, class Period1, class Rep2, class Period2>
constexpr bool operator==(const duration<Rep1, Period1>& lhs,
const duration<Rep2, Period2>& rhs);
template<class Rep1, class Period1, class Rep2, class Period2>
constexpr bool operator<(const duration<Rep1, Period1>& lhs,
const duration<Rep2, Period2>& rhs);
template<class Rep1, class Period1, class Rep2, class Period2>
constexpr bool operator>(const duration<Rep1, Period1>& lhs,
const duration<Rep2, Period2>& rhs);
template<class Rep1, class Period1, class Rep2, class Period2>
constexpr bool operator<=(const duration<Rep1, Period1>& lhs,
const duration<Rep2, Period2>& rhs);
template<class Rep1, class Period1, class Rep2, class Period2>
constexpr bool operator>=(const duration<Rep1, Period1>& lhs,
const duration<Rep2, Period2>& rhs);