eXSLT date:add template

I recently had a need to do some date calculations in XSLT (1.0) and I 
thought it would be a good opportunity to take a look at the eXSLT function 
and template library.
What I wanted to do was, given a start date (lets call it startDate), 
calculate a new date (lets call it endDate) to be startDate + 1 year - 1 
day.
I settled on calling the date:add template twice, the first time to add the 
duration P1Y and the second time to subtract the day from the result of the 
first call (duration -P1D).
This works fine until we get to those pesky leap year dates (and even for 
these I'm not saying the implementation is wrong - I am just looking for 
clarification of the design). The date that I have a question over is (not 
surprisingly) the 29th February !. An example :-
Note: 2004 is a leap year.

The input doc looks like this :-

<eXLST_dateAdd_Test1>
   <startDate>2004-02-29</startDate>
</eXLST_dateAdd_Test1>

and the output like this :-

<eXLST_dateAdd_Test1>
   <startDate>2004-02-29</startDate>
   <currentDatePlus1Year>2005-02-28</currentDatePlus1Year>
   <endDate>2005-02-27</endDate>
</eXLST_dateAdd_Test1>

Here endDate is shown as the 27th Feb. One might expect this to be the 28th, 
but it all depends on whether adding 1 year to the 29th Feb should be 28th 
Feb in the following year or 01 March. The implementation takes the position 
that it should be 28th Feb and therefore, subtracting 1 day yields 27th.
This seems reasonable.

BUT, if I use a startDate of 2004-2-28 I will also get the same result. H'mm 
now I'm not so sure, maybe adding 1 year to 29th February SHOULD be 01 March 
after all ??
Comments

Fraser.

Here's the simple XSLT I was using to test with :-

<?xml version="1.0" encoding="UTF-8"?>
<xsl:stylesheet version="1.0" 
xmlns:xsl="http://www.w3.org/1999/XSL/Transform"; 
xmlns:date="http://exslt.org/dates-and-times"; 
extension-element-prefixes="date">
   <xsl:import href="../functions/add/date.add.template.xsl"/>
   <xsl:output method="xml" version="1.0" encoding="UTF-8" indent="yes"/>
   <xsl:template match="*|@*|text()">
       <xsl:copy>
           <xsl:apply-templates select="*|@*|text()|comment()"/>
       </xsl:copy>
   </xsl:template>
   <xsl:template match="/">
       <xsl:apply-templates/>
   </xsl:template>
   <xsl:template match="startDate">
       <xsl:copy-of select="."/>
           <!-- TASK: Add 1 Year minus 1 day -->
           <!-- Add 1 Year -->
           <xsl:variable name="currentDatePlus1Year">
               <xsl:call-template name="date:add">
                   <xsl:with-param name="date-time" select="."/>
                   <xsl:with-param name="duration" select="'P1Y'"/>
               </xsl:call-template>
           </xsl:variable>
          <currentDatePlus1Year><xsl:value-of 
select="$currentDatePlus1Year"/></currentDatePlus1Year>
           <!-- Subtract 1 Day -->
           <xsl:variable name="endDate">
               <xsl:call-template name="date:add">
                   <xsl:with-param name="date-time" 
select="$currentDatePlus1Year"/>
                   <xsl:with-param name="duration" select="'-P1D'"/>
               </xsl:call-template>
           </xsl:variable>
           <endDate>
               <xsl:value-of select="$endDate"/>
           </endDate>
   </xsl:template>
</xsl:stylesheet>



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