date arithmetic

Hi,

I want to add 6 months to a date (in xslt2.0) but cant find any 
references on how to do this.  After much googling I  tried something like:
  <xsl:value-of select="xs:date($reviewDate) + 
xs:duration('P0Y0M6DT0H00M')"/>
  
But the processor (Saxon 8.0)  says that date arithmetic is not 
supported on xs:duration.   If I am barking up the right tree, then what 
are the supported subtypes?  Hope someone can help,
David Sinclair