title: How to add duration to time_point?
body: <p>I tried something like this:</p>
  
  <pre><code>using clock = std::chrono::system_clock;
  clock::time_point nowp = clock::now();
  clock::time_point end = nowp + std::chrono::seconds(10);
  time_t nowt =  clock::to_time_t ( nowp );
  time_t endt =  clock::to_time_t ( end);
  std::cerr  &lt;&lt; " " &lt;&lt; ctime(&amp;nowt) &lt;&lt; " " &lt;&lt; ctime(&amp;endt) &lt;&lt; std::endl;
  </code></pre>
  
  <p>But it prints:</p>
  
  <pre><code>Sat Dec 16 15:06:43 2017
  Sat Dec 16 15:06:43 2017
  </code></pre>
  
  <p>What am I doing wrong here? How do add ten seconds to now?</p>