I am unable to use any of the date functions. I keep receiving errors along the lines of :

expr.SetContext(ctxt)' threw an exception of type 'System.Xml.XPath.XPathException'
base {System.SystemException}: {"Extension function not found: add"}
Message: "Extension function not found: add"

Here is the source code that reproduces it when clean installed (adapted from this MSDN article, with a new date field called launchedDate added)

public static string datequery = "date:add(/books/@launchedTime, 'P10D')";

private static void TestXPathDocument()

Console.WriteLine("BEGIN: Testing XPathDocument...");
XPathDocument doc = new XPathDocument(@"c:\temp\ConsoleApplication1\ConsoleApplication1\books.xml");
XPathNavigator nav;
XPathExpression expr;
ExsltContext ctxt;
nav = doc.CreateNavigator();
expr = nav.Compile(datequery);
ctxt = new ExsltContext(nav.NameTable);
expr.SetContext(ctxt);
Console.WriteLine("Date function result {0}", nav.Evaluate(expr));
Console.WriteLine("END: Testing XPathDocument...");
}

Thanks Oleg, that did the trick,

The other question I had was to do with the data type the method returns. You get back a duration data type, which is very nice, however functions for manipulating Duration's are only available in XPath 2.0. What I wanted to do is if the duration is greater than 4 days, then do something.

The function days-from-duration would be ideal. How do you deal with duration data types without these kind of functions?

Is it the intention of MvpXml to include some XPath 2.0 functionality or only EXSLT?