std::chrono::system_clock::from_time_t(3)

NAME
std::chrono::system_clock::from_time_t - std::chrono::system_clock::from_time_t

Synopsis
   static std::chrono::system_clock::time_point from_time_t( std::time_t (since C++11)
   t ) noexcept;
   Converts t to a time point type, using the coarser precision of the two types.
   If time_point has lower precision, it is implementation defined whether the value is
   rounded or truncated.

Parameters
   t - std::time_t value to convert

Return value
   A value of type std::chrono::system_clock::time_point representing t.

Example
// Run this code
 #include <chrono>
 #include <ctime>
 #include <iostream>
 #include <thread>
 int main()
 {
     using namespace std::chrono_literals;
     const std::time_t t = std::time(nullptr); // usually has "1 second" precision
     const auto from = std::chrono::system_clock::from_time_t(t);
     std::this_thread::sleep_for(500ms);
     const auto diff = std::chrono::system_clock::now() - from;
     std::cout << diff << " ("
               << std::chrono::round<std::chrono::milliseconds>(diff)
               << ")\n";
 }

Possible output:
 987654321ns (987ms)

See also
   to_time_t converts a system clock time point to std::time_t
   [static] (public static member function)