title: Change date of a DateTimeIndex
body: <p>I have a csv file named <code>data.csv</code> such as </p>
  
  <pre><code>TS;val
  10:00;0.1
  10:05;0.2
  10:10;0.3
  10:15;0.4
  </code></pre>
  
  <p>I read this csv file using this script</p>
  
  <pre><code>#!/usr/bin/env python
  import pandas as pd
  
  if __name__ == "__main__":
      yyyy = 2013
      mm = 2
      dd = 1
  
      df = pd.read_csv('data.csv', sep=';', parse_dates=[0], index_col=0)
  
      print(df)
  </code></pre>
  
  <p>I get this</p>
  
  <pre><code>                     val
  TS                      
  2013-06-17 10:00:00  0.1
  2013-06-17 10:05:00  0.2
  2013-06-17 10:10:00  0.3
  2013-06-17 10:15:00  0.4
  </code></pre>
  
  <p>I would like to change date of every DateTimeIndex to 2013-02-01</p>
  
  <pre><code>                     val
  TS                      
  2013-02-01 10:00:00  0.1
  2013-02-01 10:05:00  0.2
  2013-02-01 10:10:00  0.3
  2013-02-01 10:15:00  0.4
  </code></pre>
  
  <p>What is the easier way to do this ?</p>