title: Previous month datetime pandas
body: <p>I have a datetime instance declared as follows:</p>
  
  <pre><code>dtDate = datetime.datetime(2016,1,1,0,0)
  </code></pre>
  
  <p>How do I get the previous month and previous year from <code>dtDate</code>?</p>
  
  <p>e.g. something like:</p>
  
  <pre><code>dtDate.minusOneMonth()
  # to return datetime.datetime(2015,12,1,0,0)
  </code></pre>