How to group items by date range in XSLT?

I have a bunch of data that looks a little like this:

<item>
  <colour>Red</colour>
  <date_created>2009-10-10 12:01:55</date_created>
  <date_sold>2009-10-20 22:32:12</date_sold>
</item>
<item>
  <colour>Blue</colour>
  <date_created>2009-11-01 13:21:00</date_created>
  <date_sold>2009-11-21 12:32:12</date_sold>
</item>
<item>
  <colour>Blue</colour>
  <date_created>2009-10-29 21:23:02</date_created>
  <date_sold>2009-10-20 02:02:22</date_sold>
</item>
<item>
  <colour>Red</colour>
  <date_created>2009-11-02 09:11:51</date_created>
  <date_sold>2009-11-20 09:15:53</date_sold>
</item>
<item>
  <colour>Red</colour>
  <date_created>2009-10-18 11:00:55</date_created>
  <date_sold>2009-10-20 11:12:22</date_sold>
</item>

Now what I would like to be able to do is to run that through an XSLT stylesheet such that I get ouput looking like this:

Colour | In stock 1 week | In stock 2 weeks | In stock 3 weeks
Red    |       1         |        3         |       2
Blue   |       0         |        2         |       1

Currently I have a stylesheet that uses basic muenchian grouping to show that 30% of stock was Red and 70% blue, but I can't see 开发者_开发技巧a way to find the number of nodes withing a given date range.

Is there a way to use keys to select a range? Do I need to create some kind of intermediate data node? Is there a different route that shows I'm barking up the wrong tree with both those suggestions? Is this even possible with XSLT or do I need to find a way to change the data source?

If you can use EXSLT date and time functions, you could use something like the following to get items by time in stock (in weeks, rounded up):

<xsl:key 
  name="items-by-weeks-in-stock" 
  match="//item"
  use="ceiling(date:seconds(date:difference(translate(date_created, ' ', 'T'),
                                            translate(date_sold, ' ', 'T')))
               div 604800)"/>