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

Question

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
}

Answer 1

It is clarifying to add output with finer units, for example:

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

For me, for the 3rd case (argument 2 seconds), the output is:

Difference = -998

(that is in milliseconds)

To analyze this, let T0 represent the time now() is first called in CheckDuration. So:

start == T0 + 2s

stop is called at T0, plus 1 second for sleeping, plus a tiny bit of processing time we can call epsilon. So:

stop == T0 + 1s + epsilon

Subtracting these two we get:

T0 + 1s + epsilon - (T0 + 2s)

simplifying:

epsilon - 1s

In my case, epsilon == 2ms

duration_cast has the behavior of truncate towards zero when the conversion can not be made exactly. So -998ms truncates to 0s. For other duration and time point rounding modes which may be helpful in your computations, see:

http://howardhinnant.github.io/duration_io/chrono_util.html