title: Tool/library for calculating intervals like "last thursday of the month"
body: <p>I'm looking for a command line tool or some sort of python library (that I can then wrap), so that I can calculate dates that are specified like "last thursday of the month".</p>
  
  <p>i.e. I want to let people enter human friendly text like that above and it should be able to calculate all the dates for any month/year/whatever that fulfil that.</p>
  
  <p>Any suggestions?</p>