title: Pandas difference in index with date values
body: <p>If I have a DataFrame like this </p>
  
  <pre><code>Date           value
  04 May 2015     1
  06 May 2015     1
  07 May 2015     1
  11 May 2015     1
  11 May 2015     1
  </code></pre>
  
  <p>How do I get the diff of the Date Index? i.e. the third col below:</p>
  
  <pre><code>Date           value   Diff
  04 May 2015     1      NA
  06 May 2015     1       2
  07 May 2015     1       1
  11 May 2015     1       4
  11 May 2015     1       0
  </code></pre>