using xsl-method 'formatDate()'

Hi,
    I am trying to format the date value in my xml file using the
'formatDate() 'method in the xsl file.

Here is what I do

<xsl:eval>formatDate(pif_date,"mm/dd/yy")</xsl:eval>


The xml file looks like this

<Customers>
    <Customer>
        <claim_company>35</claim_company> 
         <claim_number>002350010014640</claim_number> 
        <pif_date>9/20/00</pif_date> 
     </Customer>
     <Customer>
      ..
      ..
   </Customer>
   .
   .
</Customers>
I get a exception when I try to run it. 
Can anyone tell me what cud be the problem?
Thanks.
Rama.    


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