namespace std::chrono { using file_clock = see below; }
template<class Duration> static sys_time<see below> to_sys(const file_time<Duration>&); template<class Duration> static file_time<see below> from_sys(const sys_time<Duration>&);
template<class Duration> static utc_time<see below> to_utc(const file_time<Duration>&); template<class Duration> static file_time<see below> from_utc(const utc_time<Duration>&);
template<class charT, class traits, class Duration>
basic_ostream<charT, traits>&
operator<<(basic_ostream<charT, traits>& os, const file_time<Duration>& t);
template<class charT, class traits, class Duration, class Alloc = allocator<charT>>
basic_istream<charT, traits>&
from_stream(basic_istream<charT, traits>& is, const charT* fmt,
file_time<Duration>& tp, basic_string<charT, traits, Alloc>* abbrev = nullptr,
minutes* offset = nullptr);