title: std::chrono, adding duration to time_point
body: <p>I am trying to add some duration to a time_point in Qt (C++11/MinGW) and I am having trouble:</p>
  
  <p>Initialization (when the program starts):</p>
  
  <pre><code>auto program_start_time = std::chrono::system_clock::now();
  auto offline_time = std::chrono::system_clock::now();
  </code></pre>
  
  <p>...</p>
  
  <p>Some activity goes offline:</p>
  
  <pre><code>offline_mark_time = std::chrono::system_clock::now();
  </code></pre>
  
  <p>...</p>
  
  <p>When the activity resumes, I need to add the offline time to my start time:</p>
  
  <pre><code>auto now = std::chrono::system_clock::now();
  program_start_time += (now - offline_mark_time);  // &lt;- Does not seem to work
  </code></pre>
  
  <p>Even though compilation and executions is ok, the program's behavior is as if I am adding zero.</p>
  
  <p><strong>How do you add or subtract a duration to a time_point?</strong></p>