title: Pandas rolling method with data to be offset
body: <pre><code>df = pd.DataFrame({'Number': [x for x in range(10)]})
  df["rolling"] = df["Number"].rolling(3).mean()
  print(df)
  </code></pre>
  
  <p>With the above code, it will output </p>
  
  <pre><code>   Number  rolling
  0       0      NaN
  1       1      NaN
  2       2      1.0
  3       3      2.0
  4       4      3.0
  5       5      4.0
  6       6      5.0
  7       7      6.0
  8       8      7.0
  9       9      8.0
  </code></pre>
  
  <p>Therefore, the rolling method applies to the data including the current index data, e.g the first rolling mean is calculated at the 3rd position, surely I believe there is an advantage, you are always using the latest information.</p>
  
  <p>Can I actually have the rolling method apply to the data up to but not including the current index? e.g the first rolling mean should now be calculated at the 4th position, using data[0:3]?</p>
  
  <pre><code>   Number  rolling
  0       0      NaN
  1       1      NaN
  2       2      NaN
  3       3      1.0
  4       4      2.0
  5       5      3.0
  6       6      4.0
  7       7      5.0
  8       8      6.0
  9       9      7.0
  </code></pre>
  
  <p>I know I can achieve the same result if do this rolling and shift the result by 1, but I want to know how I can input the 'correct' data, if I don't want the rolling method to use the last index data.</p>