title: How to find holes in a Pandas time index
body: <p>I am interesting in knowing if a time index contains holes. Say I have the following Series</p>
  
  <pre><code>ss = pd.Series( nr.randn(3), index=[ '2014-01-01', '2014-01-02', '2014-01-03' ] )
  ss.index = pd.to_datetime( ss.index )
  ss
  </code></pre>
  
  <p>Output </p>
  
  <pre><code>2014-01-01    0.976455
  2014-01-02   -0.610322
  2014-01-03   -0.631592
  dtype: float64
  </code></pre>
  
  <p>I thought I could do what I would do with lists (<code>l[1:]-l[:-1]</code>)</p>
  
  <pre><code>ss.index[1:] - ss.index[:-1]
  </code></pre>
  
  <p>But here is the output, which I don't understand</p>
  
  <pre><code>&lt;class 'pandas.tseries.index.DatetimeIndex'&gt;
  [2014-01-03]
  Length: 1, Freq: None, Timezone: None
  </code></pre>
  
  <p>I ended up doing this (which is kinda ugly)</p>
  
  <pre><code>tmp = pd.Series( ss.index[1:] ) - pd.Series( ss.index[:-1] )
  (tmp[0] == tmp ).all()
  </code></pre>
  
  <p>So I have 2 questions:</p>
  
  <ul>
  <li>What is <code>ss.index[1:] - ss.index[:-1]</code> doing?</li>
  <li>Is there a better way to do what I am doing?</li>
  </ul>