title: How to resample 6 months
body: <p>How to resample 6 months in pandas? For example, I have 2 dates <code>'2018-02-07'</code> and <code>'2018-09-17'</code>. I want to resample them in 6 month period, the result should be <code>'2018-06-30'</code> and <code>'2018-12-31'</code>.</p>
  
  <p>I setup the code below:</p>
  
  <pre><code>import pandas as pd
  import numpy as np
  
  series = pd.Series([1,2], index=[pd.Timestamp('2018-02-07'), pd.Timestamp('2018-09-17')])
  s.resample('6M').sum()
  </code></pre>
  
  <p>It gives <code>'2018-02-28'</code>, <code>'2018-08-31'</code>, <code>'2019-02-28'</code>, but this is not what I want.</p>