27 Time library [time]

27.7 Clocks [time.clock]

27.7.5 Type file_­clock [time.clock.file]

27.7.5.2 Member functions [time.clock.file.members]

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The type denoted by file_­clock provides precisely one of the following two sets of static member functions: 
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>&);
or: 
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>&);
These member functions shall provide time_­point conversions consistent with those specified by utc_­clock, tai_­clock, and gps_­clock.
The Duration of the resultant time_­point is computed from the Duration of the input time_­point.