title: How to change a DateTimeIndex in a pandas dataframe to all the same year?
body: <p>I have data like this:</p>
  
  <pre><code>                   Charge 1     Charge 2
  observation_date                      
  1970-01-31        35.535318   0.073390
  1970-02-28        27.685739   0.050302
  
  ...
  
  2013-01-31        27.671290   0.296882
  2013-02-28        26.647262   0.225714
  2013-03-31        21.495699   0.362151
  </code></pre>
  
  <p>How do I re-index the data (the observation_date) so that all the years become 2013? </p>
  
  <p>So 1970-01-31 becomes 2013-01-31, etc. I understand that there will be many times when the index will be the same. </p>