looping in xslt

I have a big xml file that comes out of a db ( + 3Mb ).
It contains a lot of date values. My xslt transformation needs to create a
new xml file
from this dump xml file and add lines for every date just ones.
So the file may contain 100 date values but only 10 different dates

( this example has 5 dates but only 3 different dates )
<?xml version="1.0"?>
<root>
  <rec>
    <date><value>010104</value></date>
  </rec>
  <rec>
    <date><value>010104</value></date>
    <date><value>020104</value></date>
  </rec>
  <rec>
    <date><value>020104</value></date>
    <date><value>030104</value></date>
  </rec>
</root>

to find these unique dates i use
<xsl:key name="mydates" match="date" use="value"/>
and to loop 3 times i use.
<xsl:for-each select="/root/rec/date[generate-id() =
generate-id(key('mydates',value)[1])]">

I need to loop multiple times so i repeat the "for-each" multiple times in
my code.
Is there a way to avoid this. It takes a long time to find all the unique
dates again and again.

Can i store a count
<xsl:for-each select="count(/root/rec/date[generate-id() =
generate-id(key('mydates',value)[1])])">
in a variable and use this in a loop?

Or there any other solutions ? i use xlst 1.1.

Thank you, Joeri