Using xpath fn: functions in XSL stylesheets

Hi,
I would like to use the XPath functions in my XSL stylesheet. For
example like those from
http://www.w3schools.com/xpath/xpath_functions.asp#context
But I cant figure out the usage. Im trying since a few days already.
As for now Ive been trying to do it like that :
XML sample file "test.xml":

<?xml version="1.0"?>
<?xml-stylesheet type="text/xsl" href="test.xsl" ?>
<sales></sales>

XSL sample file "test.xsl":

<?xml version='1.0'?>
<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
xmlns:fn="http://www.w3.org/2005/02/xpath-functions" version="1.0">
  <xsl:template match="/">
    TEST :
    <xsl:value-of select='string(34)'/>
    <br/>
    TEST :
    <xsl:value-of select='fn:current-dateTime()'/>
    <br/>
  </xsl:template>
</xsl:stylesheet>

Ive been trying to use that with Firefox 2.0, Internet Explorer, and
XML Notepad. No success.
My system is Windows Vista. The "string(34)" works fine but when I use
the "fn:current-dateTime()" function Im getting errors like "the
function current-dateTime() not found". The same thing happens when I
try to use the other fn: functions. I think Im not specifying the
"http://www.w3.org/2005/02/xpath-functions" namespace the right way
but I cant find on the internet any working example of its usage.
Could somebody tell me please what is the right way to use those fn:
functions ? The ones I want the mose are fn:current-dateTime() and
fn:base-uri().

Thank you for any help,
Thomas