title: How to group a pandas dataframe by a defined time interval?
body: <p>I have a dataFrame like this, I would like to group every 60 minutes and start grouping at 06:30.</p>
  
  <pre><code>                           data
  index
  2017-02-14 06:29:57    11198648
  2017-02-14 06:30:01    11198650
  2017-02-14 06:37:22    11198706
  2017-02-14 23:11:13    11207728
  2017-02-14 23:21:43    11207774
  2017-02-14 23:22:36    11207776
  </code></pre>
  
  <p>I am using:</p>
  
  <pre><code>df.groupby(pd.TimeGrouper(freq='60Min'))
  </code></pre>
  
  <p>I get this grouping:</p>
  
  <pre><code>                      data
  index       
  2017-02-14 06:00:00     x1
  2017-02-14 07:00:00     x2
  2017-02-14 08:00:00     x3
  2017-02-14 09:00:00     x4
  2017-02-14 10:00:00     x5
  </code></pre>
  
  <p>but I am looking for this result:</p>
  
  <pre><code>                      data
  index       
  2017-02-14 06:30:00     x1
  2017-02-14 07:30:00     x2
  2017-02-14 08:30:00     x3
  2017-02-14 09:30:00     x4
  2017-02-14 10:30:00     x5
  </code></pre>
  
  <p>How can I tell the function to start grouping at 6:30 at one-hour intervals?</p>
  
  <p>If it can not be done by the <em>.groupby(pd.TimeGrouper(freq='60Min'))</em>, how is the best way to do it?</p>
  
  <p>A salute and thanks very much in advance</p>