title: TimeGrouper, pandas
body: <p>I use <code>TimeGrouper</code> from <code>pandas.tseries.resample</code> to sum monthly return to 6M as follows:</p>
  
  <pre><code>6m_return = monthly_return.groupby(TimeGrouper(freq='6M')).aggregate(numpy.sum)
  </code></pre>
  
  <p>where <code>monthly_return</code> is like:</p>
  
  <pre><code>2008-07-01    0.003626
  2008-08-01    0.001373
  2008-09-01    0.040192
  2008-10-01    0.027794
  2008-11-01    0.012590
  2008-12-01    0.026394
  2009-01-01    0.008564
  2009-02-01    0.007714
  2009-03-01   -0.019727
  2009-04-01    0.008888
  2009-05-01    0.039801
  2009-06-01    0.010042
  2009-07-01    0.020971
  2009-08-01    0.011926
  2009-09-01    0.024998
  2009-10-01    0.005213
  2009-11-01    0.016804
  2009-12-01    0.020724
  2010-01-01    0.006322
  2010-02-01    0.008971
  2010-03-01    0.003911
  2010-04-01    0.013928
  2010-05-01    0.004640
  2010-06-01    0.000744
  2010-07-01    0.004697
  2010-08-01    0.002553
  2010-09-01    0.002770
  2010-10-01    0.002834
  2010-11-01    0.002157
  2010-12-01    0.001034
  </code></pre>
  
  <p>The 6m_return is like:</p>
  
  <pre><code>2008-07-31    0.003626
  2009-01-31    0.116907
  2009-07-31    0.067688
  2010-01-31    0.085986
  2010-07-31    0.036890
  2011-01-31    0.015283
  </code></pre>
  
  <p>However I want to get the <code>6m_return</code> starting 6m from 7/2008 like the following:</p>
  
  <pre><code>2008-12-31    ...
  2009-06-31    ...
  2009-12-31    ...
  2010-06-31    ...
  2010-12-31    ...
  </code></pre>
  
  <p>Tried the different input options (i.e. loffset) in TimeGrouper but doesn't work.
  Any suggestion will be really appreciated!</p>