DouKaku?

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  xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
  xmlns:xs="http://www.w3.org/2001/XMLSchema"
  xmlns:fn="http://www.w3.org/2005/xpath-functions"
  xmlns:my="uri:ja.doukaku.org:my-functions"
  exclude-result-prefixes="my"
  >

  <xsl:output method="text" />

  <xsl:template match="/" >
    <xsl:value-of select="my:DateEx('2008-09-01T10:02:38', -1)"/>
  </xsl:template>

  <xsl:function name="my:DateEx" as="xs:string">
    <xsl:param name="dstr" as="xs:string" />
    <xsl:param name="sec" as="xs:integer" />

    <xsl:sequence select="xs:string(
      xs:dateTime($dstr) +
      xs:dayTimeDuration(
        fn:concat(
          (if($sec&lt;0)then'-'else''),
          'PT', xs:string(fn:abs($sec)), 'S'
          )
        )
      )" />
  </xsl:function>

</xsl:stylesheet>