title: Sum values from month
body: <p>I have a table of values, dates and categories:</p>
  
  <pre><code>5   1.09.2013   red
  -7  2.09.2013   red
  9   21.09.2013  red
  -2  3.10.2013   red
  11  28.09.2013  green
  3   2.10.2013   green
  8   16.10.2013  green
  </code></pre>
  
  <p>I calculate sum of values where category is green with:</p>
  
  <pre><code>=sum(filter(A:A;C:C="green"))
  </code></pre>
  
  <p>How to sum just values from October where category is green ?</p>