CustomDate custDate = new CustomDate() ;

<xsl:transform
        version="1.0"
        xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
        xmlns:myCustDate="urn:custDate">

xslArgs.AddExtensionObject("urn:custDate", custDate) ;

<xsl:value-of select="myCustDate:GetDateDiff(./joiningdate)"/>

using System ;
using System.IO ;
using System.Xml ;
using System.Xml.Xsl ;
using System.Xml.XPath ;

public class XsltExtension{

    public static void Main(string[] args){
        
        if (args.Length == 2){
            
            Transform(args[0], args[1]) ;
            
        }else{
            
            PrintUsage() ;
            
        }
    }
    
    public static void Transform(string sXmlPath, string sXslPath){
        
        try{
            
            //load the Xml doc
            XPathDocument myXPathDoc = new XPathDocument(sXmlPath) ;

            XslTransform myXslTrans = new XslTransform() ;
            
            //load the Xsl 
            myXslTrans.Load(sXslPath) ;
            
            XsltArgumentList xslArgs = new XsltArgumentList() ;
            
            //create custom object
            CustomDate custDate = new CustomDate() ;
            
            //pass an instance of the custom object
            xslArgs.AddExtensionObject("urn:custDate", custDate) ;
                        
            //create the output stream
            XmlTextWriter myWriter = new XmlTextWriter("extendXSLT.html", null) ;
            
            //pass the args,do the actual transform of Xml
            myXslTrans.Transform(myXPathDoc,xslArgs, myWriter) ;        

            myWriter.Close() ;

        }catch(Exception e){

            Console.WriteLine("Exception: {0}", e.ToString());
        }
        
    }
    
    public static void PrintUsage(){
        Console.WriteLine("Usage: XsltExtension.exe <xml path> >xsl path<") ;
    }

}

//our custom class
public class CustomDate{
    
    //function that gets called from XSLT
    public string GetDateDiff(string xslDate){
        
        DateTime dtDOB = DateTime.Parse(xslDate) ;
        
        DateTime dtNow = DateTime.Today ;
        
        TimeSpan tsAge = dtNow.Subtract(dtDOB) ;
        
        return tsAge.Days.ToString() ;
    }

}