The following code example is taken from the book
C++17 - The Complete Guide by Nicolai M. Josuttis, Leanpub, 2017
The code is licensed under a Creative Commons Attribution 4.0 International License. Creative Commons License

// raw code

#include <string>
#include <chrono>
#include <filesystem>

std::string asString(const std::filesystem::file_time_type& tp)
  using system_clock = std::chrono::system_clock;
  auto t = system_clock::to_time_t(tp + (system_clock::now()
                                          - decltype(tp)::clock::now()));
  // convert to calendar time (including skipping trailing newline):
  std::string ts = std::ctime(&t);
  return ts;