title: Calculate maximum sum of values over any 30 days period
body: <p>I have data as follows:</p>
  
  <pre><code>Date        Value
  2011-01-01  1
  2011-01-02  5
  2011-01-03  30
     .....
  2012-01-01  4
  </code></pre>
  
  <p>I want to calculate which 30 day period within my data that has the maximum sum of values.</p>
  
  <p>Not sure if my question makes sense as it's quite difficult to explain as I don't have a good maths head.</p>
  
  <p>Thanks
  Steve</p>