Cannot run datetime extensions

Hi,

I am trying to use some date and time functions in my XSL but when I do, I get
this error:

For extension function, could not find method
org.apache.xalan.lib.ExsltDatetime.dateTime([ExpressionContext,] #NODESET).

My XSL declaration looks like this:

<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
  xmlns:redirect="http://xml.apache.org/xalan/redirect"
  xmlns:datetime="http://exslt.org/dates-and-times"
  extension-element-prefixes="redirect datetime" version="1.0">

and my usage like this:

<xsl:template match="DATAFILE">
  <xsl:variable name="nextUpdate">
    <xsl:value-of select="datetime:date-time(@GENTIME)" />
  </xsl:variable>
  <NextUpdate>
    <xsl:value-of select="nextUpdate" />
  </NextUpdate>
</xsl:template>

What am I doing wrong?

Thanks,

L