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std::chrono::system_clock:: from_time_t

From cppreference.net
C++
Date and time library
std::chrono::system_clock
static std :: chrono :: system_clock :: time_point from_time_t ( std:: time_t t ) noexcept ;
(since C++11)

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.

Contents

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

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