title: Excel VLOOKUP equivalent in pandas
body: <p>I have following dataframe:</p>
  
  <pre><code>               A           B            C
   Index
  2001-06-30    100       2001-08-31     (=value of A at date B)
  2001-07-31    200       2001-09-30      ...
  2001-08-31    300       2001-10-31      ...
  2001-09-30    400       2001-11-30      ...
  </code></pre>
  
  <p>Column <code>B</code> consists of dates from column <code>A</code> shifted forward by some. I would like to generate column <code>C</code> that consists of the values from column <code>A</code> on date <code>B</code>. (preferably in the logic the Excel VLOOKUP formula would do it. I am not looking for simply <code>shift(-2)</code> here because in reality the shift between <code>B</code> and <code>Index</code> is not always equal).</p>
  
  <p>I tried <code>df.loc['B', 'A']</code> but this most probably too simplistic and produced an error.</p>