title: Split CSV by date
body: <p>I'm hoping that there is a relatively simple solution to my problem:</p>
  
  <p>I have a csv with a selection of data points but they all include a date field.</p>
  
  <p>I would like to be able to split up the csv into multiple files based on the month of the date field.</p>
  
  <p>For example:  I would like to be able to have all the records before March 2015 in 1 file, all before April 2015 in another, up to all before October 2016 etc.</p>
  
  <p>In this case there will be many duplicate records between the files.</p>
  
  <p>Is there a way to do this with a simple bit of python code or is there an easier method?</p>
  
  <p>Thanks in advance</p>