This is known but I don't see it listed as a bug needing fixing. The new sortable table
will sort dates but only if the dates are in the uncommon YYYY-MM-DD format and the
user's preference is also set to display dates that date format.
This should be made sortable for any of the supported date formats and for any
combination of supported date input formats and display formats.
http://meta.wikimedia.org/wiki/Help:Sorting#Dates discusses issue with a hack to get