title: how to convert "std::chrono::system_clock::now()" to double
body: <pre><code>auto current_time = std::chrono::system_clock::now();
  </code></pre>
  
  <p>Using <code>std::chrono</code> in <code>C++</code> I get current time like above.<br>
  <strong>How can I use it to get the number of seconds since the clock's epoch as a  <code>double</code>?</strong></p>