title: Select only the rows which timestamp correspond to the current month
body: <p>I am starting to try some experiments using Google SpreadSheets as a DB and for that I am collecting data from different sources and inserting them via spreadsheets API into a sheet. </p>
  
  <p>Each row has a value (Column B) and a timestamp (Column A).</p>
  
  <pre><code>+---------------------+------+
  |        ColA         | ColB |
  +---------------------+------+
  | 13/10/2012 00:19:01 |   42 |
  | 19/10/2012 00:29:01 |  100 |
  | 21/10/2012 00:39:01 |   23 |
  | 22/10/2012 00:29:01 |    1 |
  | 23/10/2012 00:19:01 |   24 |
  | 24/10/2012 00:19:01 |    4 |
  | 31/10/2012 00:19:01 |    2 |
  +---------------------+------+
  </code></pre>
  
  <p>What I am trying to do is to programmatically add the sum of all rows in Column B where Column A is equal to the current month into a different cell.</p>
  
  <p>Is there any function that I can use for that? Or anyone can point me to the right direction on how can I create a custom function which might do something like this? I know how to do this using MySQL but I couldn't find anything for Google SpreadSheets</p>
  
  <p>Thanks in advance for any tip in the right direction.</p>