getting distinct sorted date values ignoring datetime - xslt-1.0

Question

I have the following xml / xsl:
<?xml version="1.0" encoding="ISO-8859-1"?>
<?xml-stylesheet type="text/xsl" href="./test.xsl"?>
<a>
<b>
<c >
<no>1</no>
<d>
<e>
<f>20060603190000</f>
</e>
</d>
</c>
<c >
<no>1</no>
<d>
<e>
<f>20060603190000</f>
</e>
</d>
</c>
<c>
<no>2</no>
<d>
<e>
<f>20060603190000</f>
</e>
</d>
</c>
<c >
<no>1</no>
<d>
<e>
<f>20060819200000</f>
</e>
<e>
<f>20060902200000</f>
</e>
</d>
</c>
<c >
<no>1</no>
<d>
<e>
<f>20070819200000</f>
</e>
<e>
<f>20070819200003</f>
</e>
<e>
<f>20070819200001</f>
</e>
<e>
<f>20060903100000</f>
</e>
</d>
</c>
<c >
<no>1</no>
<d>
<e>
<f>20060819200000</f>
</e>
<e>
<f>20060902200000</f>
</e>
</d>
</c>
</b>
</a>
<?xml version="1.0" encoding="ISO-8859-15"?>
<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
version="1.0" >
<xsl:output method="text" indent="yes" />
<xsl:template match="/">
<table>
<tr>
<xsl:for-each select="a/b/c[no=1]/d/e/f[not(.=preceding::*)]" ><xsl:sort select="." data-type="number"/> 
<td><xsl:value-of select="."/></td>
</xsl:for-each>
</tr>
</table>
</xsl:template>
</xsl:stylesheet>
when applied, the result looks like this:
20060603190000
20060819200000
20060902200000
20060903100000
20070819200000
20070819200001
20070819200003
which is partly correct, since:
i want to display distinct values of f (which works)
i want to sort the values of f (which works too)
but:
i want to display values that are distinct for the first 8 digits only (year, month, day), so that the result would be:
20060603190000
20060819200000
20060902200000
20060903100000
20070819200000
ie values with the same date but different time are seen as equal.
I have tried to use substring(values, 8) in different places, but i can't get it to work that way nor using key/generate-id.
Can anyone please give me advise on that?
edited:
i have solved the problem on my own.
the following template create a string containing all dates sorted by time and truncated to the first 8 digits (the date withaout the time):
<xsl:template match="/">
<xsl:variable name="text">
<xsl:for-each select="a/b/c[no=1]/d/e/f[not(.=preceding::*)]" ><xsl:sort select="." data-type="number"/>
<xsl:value-of select="substring(.,0,9)"/>-</xsl:for-each>
</xsl:variable>
text:<xsl:value-of select="$text"/>
<xsl:call-template name="filter"><xsl:with-param name="text"><xsl:value-of select="$text"/></xsl:with-param><xsl:with-param name="previousElement"/></xsl:call-template >
</xsl:template>
output example: 20060603-20060819-20060902-20060902-20060902-20060903-20070819-20070819-20070819-20110902-20110902-
then a second template is called which outputs the distinct values by recursivly calling itself:
<xsl:template name="filter">
<xsl:param name="text"/>
<xsl:param name="previousElement"/>
<xsl:variable name="firstElement"><xsl:value-of select="substring-before($text, '-')"/></xsl:variable>
<xsl:variable name="rest"><xsl:value-of select="substring-after($text, '-')"/></xsl:variable>
<!-- firstElement:<xsl:value-of select="$firstElement"/>
previousElement:<xsl:value-of select="$previousElement"/>
rest:<xsl:value-of select="$rest"/>
-->
<xsl:if test="string-length($firstElement) > 0">
<xsl:choose>
<xsl:when test="$firstElement = $previousElement">
<xsl:call-template name="filter"><xsl:with-param name="text"><xsl:value-of select="$rest"/></xsl:with-param><xsl:with-param name="previousElement"><xsl:value-of select="$firstElement"/></xsl:with-param></xsl:call-template >
</xsl:when>
<xsl:otherwise>
output:<xsl:value-of select="$firstElement"/>
<xsl:call-template name="filter"><xsl:with-param name="text"><xsl:value-of select="$rest"/></xsl:with-param><xsl:with-param name="previousElement"><xsl:value-of select="$firstElement"/></xsl:with-param></xsl:call-template >
</xsl:otherwise>
</xsl:choose>
</xsl:if>
</xsl:template>

Answer 1

i answered my question myself as seen above. according to the advice of Marcus Rickert i will answer it this way and accept it to make it closable and maybe helpful for others.