Sibling sort after grouping

I have a body of content, structured as:

<content>
 <item>
  <tag>...</tag>
  <tag>...</tag>
  <tag>...</tag>
  <date>...</date>
  ...
 </item>
 ...
</content>

My goal is to generate a list of all tags, sortable by date. "Date" in
this context would mean the most recent "date" child of any "item" node
that contains a given "tag" child. Dates are ISO 8601 formatted.

Currently, this list of tags is sorted alphabetically. How can I modify
the below XSL (1.0) to achieve a "last-modified" sort, as outlined above?

<xsl:key name="group-by-tag" select="content/item/tag" use="."/>
...
<xsl:for-each
select="content/node/tag[generate-id()=generate-id(key('group-by-tag',.)[1])]">
 <xsl:sort select="." order="ascending"/>
 <xsl:value-of select="."/>
</xsl:for-each>

Thank you for any assistance you can provide.