title: Pandas time series add column showing 1 hour intervals
body: <p>I can create an hour of day column in Pandas using <code>data['hod'] = [r.hour for r in data.index]</code> which is really useful for groupby related analysis.  However, I would like to be able to create a similar column for 1 hour intervals starting at 09:30 instead of 09:00. So the column values would be 09:30-10:30, 10:30-11:30 etc.</p>
  
  <p>The aim is to be able to groupby these values in order to gain stats for the time period.</p>
  
  <p>Using data as follows.  I already added hour of day, day of week etc, I just need the same for time sliced from 09:30 onwards in one hour intervals:</p>
  
  <pre><code>data['2008-05-06 09:00:00':].head()
  
  
  Open    High    Low Last    Volume  hod dow dom minute
  Timestamp                                   
  2008-05-06 09:00:00 1399.50 1399.50 1399.25 1399.50 4   9   1   6   0
  2008-05-06 09:01:00 1399.25 1399.75 1399.25 1399.50 5   9   1   6   1
  2008-05-06 09:02:00 1399.75 1399.75 1399.00 1399.50 19  9   1   6   2
  2008-05-06 09:03:00 1399.50 1399.75 1398.50 1398.50 37  9   1   6   3
  2008-05-06 09:04:00 1398.75 1399.00 1398.75 1398.75 15  9   1   6   4
  </code></pre>