title: Round down datetime to previous hour
body: <p>How to round down datetime to previous hour? for example:</p>
  
  <pre><code>print datetime.now().replace(microsecond=0)
  &gt;&gt; 2017-01-11 13:26:12.0
  </code></pre>
  
  <p>round down to previous hour: <code>2017-01-11 12:00:00.0</code></p>