title: Pandas Datetime multi-index
body: <p>For the following dataframe, each account may have a different rate each month.  I was trying to locate the rate to be used for a particular transaction.</p>
  
  <p>E.g. if I am looking for the Rate_A for account one at 2018-01-20, I should be getting the -1.206412 value.</p>
  
  <p>What would be the best way to locate this value?  I tried to use resample('D').ffill(), but get an error instead as it doesn't seem to work on multi-index.</p>
  
  <p>Thanks</p>
  
  <pre><code>                    Rate_A    Rate_B    Rate_C
  date     account                              
  2018-01   one    -1.206412  0.132003  1.024180
            two     2.565646 -0.827317  0.569605
  2018-02   one     1.431256 -0.076467  0.875906
            two     1.340309 -1.187678 -2.211372
  </code></pre>