var startDate = $("#startDate").data("kendoDatePicker").value();
var endDate = $("#endDate").data("kendoDatePicker").value();
var diff = Math.round((endDate - startDate)/1000/60/60/24); //Difference in days
In the attachments you can find a sample page where you could see this approach in action.
the Telerik team
Join us on our journey to create the world's most complete HTML 5 UI Framework - download Kendo UI now!