title: how to do forward rolling sum in pandas?
body: <p>I have this dataframe:</p>
  
  <pre><code>dates = pd.date_range(start='2016-01-01', periods=20, freq='d')
  df = pd.DataFrame({'A': [1] * 20 + [2] * 12 + [3] * 8,
                     'B': np.concatenate((dates, dates)),
                     'C': np.arange(40)})
  </code></pre>
  
  <p>I sorted the data frame by Date:</p>
  
  <pre><code>df.sort_values('B',inplace=True)
  </code></pre>
  
  <p>I am looking to do a forward rolling sum on date. However, I can only do backward rolling sum using:</p>
  
  <pre><code>df.groupby('A').rolling(7, on='B',min_periods=0).C.sum()
  
  A  B         
  1  2016-01-01      0.0
     2016-01-02      1.0
     2016-01-03      3.0
     2016-01-04      6.0
     2016-01-05     10.0
     2016-01-06     15.0
  </code></pre>
  
  <p>I want to do forward rolling sum.</p>