How to implement date difference in XSLT 1.0

Hi Guys...need your help in below explained XML to CSV transformation.

Input:
-------
<Result>
   <Timesheet>
    <ID>888001</ID>
    <Name>Abhi</Name>
    <Week_Ending_Date>9/1/09</Week_Ending_Date>
    <Dept>Test Department</Dept>
    <Hours_Basic_Saturday>5:00</Hours_Basic_Saturday>
    <Hours_Basic_Sunday>0:00</Hours_Basic_Sunday>
    <Hours_Basic_Monday>9:00</Hours_Basic_Monday>
    </Timesheet>
</Result>   
           
Desired Output:
-----------------
The CSV file should have multiple records depending on days worked and
--> Word date as a date value derived from 'Week_End_Date' which is always a
friday date. i.e if Week_End_Date is 02/10/2009, Work date for Monday should
be derived by something like date(Week_End_Date) - 4.

Any idea how can i implement such date functions in XSLT 1.0 to get the below
given output.

ID,     Name, Week Ending, Dept,    Work Date, Reg Hours
888001, Abhi, 2/10/09, Test Department,26/09/09 , 5:00
888001, Abhi, 2/10/09, Test Department,28/09/09 , 9:00

Intermediate XSLT:
-----------------
I'm using this xslt which gives me the above output but don't have the date
logic yet.

<xsl:stylesheet
   xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
   version="1.0">
   <xsl:output method="text"/>
   <xsl:param name="sep" select="', '"/>
   <xsl:param name="lf" select="'
'"/>

   <xsl:template match="/">
        <xsl:value-of select="concat('ID', $sep, 'Name', $sep,'Week Ending',
$sep, 'Dept', $sep,'Work Date',$sep,'Reg Hours', $lf)"/>
   <xsl:for-each select="Result/Timesheet">
   <xsl:if test="string(@Hours_Basic_Saturday) != '0.00'">
<xsl:value-of select="concat(ID, $sep, Name,$sep, Week_Ending_Date,$sep, Dept,
$sep,'',  $sep,Hours_Basic_Saturday,$lf)"/>
</xsl:if>
       
   <xsl:if test="string(@Hours_Basic_Sunday) != '0.00'">
    <xsl:value-of select="concat(ID, $sep, Name,$sep, Week_Ending_Date,$sep,
Dept, $sep,'',  $sep,Hours_Basic_Saturday, $lf)"/>
</xsl:if>
</xsl:for-each>
   </xsl:template>
</xsl:stylesheet>

Many Thanks,
Abhilash



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