I've been looking around for a way to compare dates. And it sounded like xslt can.

I'm trying like this:

<xsl:param name="date" />
<xsl:for-each select="dates/date">
<xsl:variable name="firstDate" select="concat(substring($date, 1,4),substring($date, 6,2),substring($date,9,2))"/>
<xsl:variable name="secondDate" select="concat(substring(., 1,4),substring(., 6,2),substring(.,9,2))"/>
<xsl:if test="$secondDate > $firstDate">
 someing
</xsl:if>
</xsl:for-each>

Doesn't seem to be working. Is this the right attempt?

Can't see anything obviously wrong with your code. Debug it by using xsl:message to display firstDate and secondDate.

In XSLT 2.0 you can compare dates directly, xs:date($x) > xs:date($y)

Michael Kay
http://www.saxonica.com/
Author, XSLT Programmer's Reference and XPath 2.0 Programmer's Reference