The Portable Way to Extract Date and Time from a File's mtime in C++

Extracting year, month, day, hour, minute, and second from a file's modification time in C++ is harder than it looks due to three portability layers: file_clock epochs differ across implementations (Unix epoch on libc++, Windows FILETIME on MSVC, a future 2174 epoch on GCC), conversion APIs are split (to_sys() vs to_utc()), and clock_cast was missing from libc++ until recently. Now that clock_cast is available on all three major implementations, the clean solution is to use clock_cast<system_clock> followed by year_month_day and hh_mm_ss calendar types. For display-only needs, std::format works directly on file_time_type. A manual clock correlation fallback exists for older toolchains.