Weird result with std::chrono::duration_cast for 1 second and 2 second

I am running a simple program where, I take a time_point with system_clock::now then this_thread::sleep_for(seconds(1)) and again a time_point with system_clock::now.

Now if I add some extra duration to the 1st time_point, it gives exactly the same result for 1 and 2 seconds!

Here is the demo code:

#include<iostream>
#include<chrono>
#include<thread>
using namespace std;

void CheckDuration (std::chrono::duration<int> seconds)
{
  auto start = std::chrono::system_clock::now() + seconds;
  std::this_thread::sleep_for(std::chrono::seconds(1));
  auto stop = std::chrono::system_clock::now();

  cout << "Difference = " << std::chrono::duration_cast<std::chrono::seconds>(stop-start).count() << endl;
}

int main ()
{
  CheckDuration(std::chrono::duration<int>(0));  // Difference = 1
  CheckDuration(std::chrono::duration<int>(1));  // Difference = 0 
  CheckDuration(std::chrono::duration<int>(2));  // Difference = 0  <=== ???
  CheckDuration(std::chrono::duration<int>(3));  // Difference = -1
}