title: how to start time group on a specific date pandas
body: <p>I have a df that I have grouped by datetime '12M'. I would like it to be grouped by 12M ending Dec. 31. Currently it is yearly grouping ending Jan. 31. It seems that there must be a simple way of doing this but I have been unable to find anything documenting one I am asking for. I've tried setting TimeGrouper('12M') but it won't start the counting from the beginning of the year, but rather is uses the first date index as a starting point</p>
  
  <pre><code>dfy = dfy.groupby([pd.TimeGrouper('12M'), 'fec_id', 'trans_typ', 'cmte_id'])['amount'].sum()
  dfy.head()
  #dfy.to_csv('out.csv')
  
  date        fec_id     trans_typ  cmte_id  
  2007-01-31  C00002600  24K        C00000729    1000
                                    C00002840    1000
                                    C00004325    1000
                                    C00005157    1000
                                    C00009985    1000
  Name: amount, dtype: int64
  </code></pre>