group dates by month then day

Hi
 
I have a node-set which needs to be grouped on a date element. The date
element is in the XSLT2 friendly format: YYYY-MM-DDThh:mm:ss.s. I need
to group by month and then day within month. Is there a way to achieve
this in 2 without treating the dates as strings and chopping then up
with substring-before, substring-after? (I'm not against
substring-before, substring-after, it just feels like a backwards step!)
 
Many thanks.
 
--
Kevin