29 Time library [time]

29.7 Clocks [time.clock]

29.7.6 Type file_clock [time.clock.file] Overview [time.clock.file.overview]

namespace std::chrono { using file_clock = see below; }
file_clock is an alias for a type meeting the Cpp17TrivialClock requirements ([time.clock.req]), and using a signed arithmetic type for file_clock​::​rep.
file_clock is used to create the time_point system used for file_time_type ([filesystems]).
Its epoch is unspecified, and noexcept(file_clock​::​now()) is true.
[Note 1: 
The type that file_clock denotes can be in a different namespace than std​::​chrono, such as std​::​filesystem.
— end note]