First, I don't know whether your dates are in d/m/y format or m/d/y format. Choosing 10 sample dates that are all ambiguous requires skill!
Secondly, you'd be much better off using the standard ISO 8601 format yyyy-mm-dd.
You've got two problems here: parsing the dates, and then grouping by common values. For the parsing problem, you need a combination of substring-after() and substring-before(). For the grouping problem, assuming you're using XSLT 1.0, see the notes on grouping at http://www.jenitennison.com/xslt/grouping
Author, XSLT Programmer's Reference and XPath 2.0 Programmer's Reference