Whats wrong with this?!

[misbiz]

Hi, Im trying to get a difference between todays date and date in my xml file -xs:date works fine its the xs:gMonthDay im really struggling with. Error message is below

<xsl:template name="timechanges" match="timechanges	 "><xsl:param name="today" select="current-date()"/><xsl:variable name="today" select="$today cast as xs:gMonthDay"/>  <xsl:variable name="dt1" select=" (dt1)"/>   <xsl:if test="$today < xs:gMonthDay($dt1) "> TRUE<xsl:value-of select="$today - xs:gMonthDay($dt1)"/></xsl:if>  </xsl:template></xsl:stylesheet>

ERROR: XPTY0004: Arithmetic operator is not defined for arguments of types (xs:gMonthDay, xs:gMonthDay) in built-in template rule in built-in template rulenet.sf.saxon.trans.XPathException: Arithmetic operator is not defined for arguments of types (xs:gMonthDay, xs:gMonthDay) Help will be much appreciated. thanks