(This bug report has originally been made on
http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=271754 , this is a copy/paste ;
the original report was against 1.1.8, but I can reproduce on 1.1.10)

The EXSLT function date:add-duration() produces bad values on some
inputs described here.  Apply the following stylesheet to any XML
document (the XML input is ignored) using xsltproc to reproduce.

<?xml version="1.0" encoding="utf-8"?>
<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
  xmlns:date="http://exslt.org/dates-and-times" version="1.0">

  <xsl:output method="text"/>

<xsl:variable name="nl">
<xsl:text>
</xsl:text>
</xsl:variable>

  <xsl:template match="/">
    <xsl:variable name="myduration1" select="'-P28DT23H55M8S'"/>
    <xsl:variable name="myduration2" select="'-P1DT12H54M59S'"/>
    <xsl:value-of select="date:add-duration($myduration1, $myduration2)"/>
    <xsl:value-of select="$nl"/>
  </xsl:template>

</xsl:stylesheet>

The buggy output from libxslt1.1 is:
-P31DT11H9M53S

The correct output should be:
-P30DT12H50M7S