title: Add time duration to C++ timepoint
body: <p>I have a starting timepoint in milliseconds like so:</p>
  
  <pre><code>using namespace std::chrono;
  typedef time_point&lt;system_clock, milliseconds&gt; MyTimePoint;
  
  MyTimePoint startTimePoint = time_point_cast&lt;MyTimePoint::duration&gt;(system_clock::time_point(steady_clock::now()));
  </code></pre>
  
  <p>Now I will have a certain number of hours that I want to add or subtract to the startTimePoint.</p>
  
  <pre><code>int numHours = -5//or 5 etc (Can be a plus or minus number)
  </code></pre>
  
  <p>How can I add this abount of time to the original startTimePoint??</p>