title: Pandas DateOffset, step back one day
body: <p>I try to understand why </p>
  
  <pre><code>print(pd.Timestamp("2015-01-01") - pd.DateOffset(day=1))
  </code></pre>
  
  <p>does not result in</p>
  
  <pre><code>pd.Timestamp("2014-12-31")
  </code></pre>
  
  <p>I am using Pandas 0.18. I run within the CET timezone.</p>