title: why is std::chrono::duration based on seconds
body: <p>I'm learning <code>&lt;chrono&gt;</code> library, and considering the <code>std::chrono::duration</code> class, is there any specific reason to base it on seconds? For example a variable to store seconds would be</p>
  
  <pre><code>chrono::duration&lt;int&gt; two_seconds(2);
  </code></pre>
  
  <p>and all other time spans require relating them to seconds, like</p>
  
  <pre><code>chrono::duration&lt;int, ratio&lt;60&gt;&gt; two_minutes(2);
  chrono::duration&lt;int, ratio&lt;1, 1000&gt;&gt; two_milliseconds(2);
  chrono::duration&lt;int, ratio&lt;60 * 60 * 24&gt;&gt; two_days(2); 
  </code></pre>
  
  <p>Are there any reasons to base duration on seconds and not on minutes, hours, etc.? </p>