template<class charT, class traits, class Rep, class Period>
  basic_ostream<charT, traits>&
    operator<<(basic_ostream<charT, traits>& os, const duration<Rep, Period>& d);
template<class charT, class traits, class Rep, class Period, class Alloc = allocator<charT>>
  basic_istream<charT, traits>&
    from_stream(basic_istream<charT, traits>& is, const charT* fmt,
                duration<Rep, Period>& d,
                basic_string<charT, traits, Alloc>* abbrev = nullptr,
                minutes* offset = nullptr);