title: Pandas .sub() works, but subtract operator fails
body: <p>I want to subtract columns of of type <code>datetime</code> in a Pandas dataframe, <code>s</code>.</p>
  
  <pre><code>[in] s.shape
  [out] (1381,5)
  </code></pre>
  
  <p>Dates in column <code>t1</code>:</p>
  
  <pre><code>[in] s['t1'].tail(3)
  [out] 56   2013-01-01
        62   2013-01-01
        63   2013-01-01
        Name: t1, dtype: datetime64[ns]
  </code></pre>
  
  <p>Dates in column <code>t2</code>:</p>
  
  <pre><code>[in] s['t2'].tail(3)
  [out] 56   2013-01-02
        62   2013-01-02
        63   2013-01-02
        Name: t2, dtype: datetime64[ns]
  </code></pre>
  
  <p>The <code>.sub()</code> operator produces the expected result: <code>d=s['t2'].sub(s['t1'])</code>.</p>
  
  <pre><code>[in] print d.shape
  [out] (1381,)
  
  [in] d.tail(3)
  [out] 56   1 days
        62   1 days
        63   1 days
        Name: t2, dtype: timedelta64[ns]
  </code></pre>
  
  <p>The subtract operator produces an unexpected result: <code>d=s['t2']-s['t1']</code>.</p>
  
  <pre><code>[in] print d.shape
  [out] (4293,)
  
  [out] 592   570 days
        593   660 days
        594   180 days
        dtype: timedelta64[ns]
  </code></pre>
  
  <p>Why?</p>