title: why does dataframe.resample('B') move data points back in time?
body: <p>This is unexpected to me:</p>
  
  <pre><code>In[18]: TimeSeries([1.0], index=[datetime(2012, 1, 1)]).resample('B')
  Out[17]: 
  2011-12-30    1
  Freq: B, dtype: float64
  </code></pre>
  
  <p>Why is that data point moved back to the 30th rather than just ignored?</p>