Sum by splitting the appended variable

Hi ..
     
     I am supposed to do an average of date-Difference . I am explicitly calculating date-difference using one of the EXSLT functions by Jeni.
    And now i need to Avg  across these Date-difference numbers. This is how i am doing presently and getting it appended in variable .

                 <xsl:variable name="dateDiffNum">
                           <xsl:for-each select="loop over Noded Set with start and End Dates ">
                                        
                                    -      Get start Date
                                    -     Get End Date
                                          
                                         <xsl:call-template name="date:difference">
                                                    Pass args
                                         </xsl:call-template>
             
                   </xsl:for-each>

                  If the number for date difference for each iteration is 5, 6 and 7 consecutively then the variable "dateDiffNums='567'"

                 I need actually 5+6+7 =18   from this variable .So i need to do some kind of splitting and write a recursive template to do the SUM ..
       
                 I have not been successful writing the template for sum of this nature ..
any ideas,,

Thanks everyone
- kumar



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