title: Convert std::chrono::time_point to unix timestamp
body: <p>How can I get an <code>std::chrono::duration</code> since a fixed date? I need this to convert a <code>std::chrono::time_point</code> to an unix timestamp. </p>
  
  <p>Insert code into XXX</p>
  
  <pre><code>auto unix_epoch_start = XXX;
  auto time = std::chrono::system_clock::now();
  auto delta = time - unix_epoc_start;
  auto timestamp = std::chrono::duration_cast&lt;std::chrono::milliseconds&gt;(delta).count();
  </code></pre>
  
  <p>I know <code>time_point</code> has a method <code>time_since_epoch()</code> but it's not guaranteed that this is the same as the unix epoch (00:00:00 UTC on 1 January 1970).</p>