I am trying to write a calculation that adds three years from a date of birth entry and then puts the result date in a different field. While I have done this with Infopath itself, I cannot seem to figure out how to select the proper node in my access datasource to put the result. Please see data structure and code below:

function msoxd__tblChild_ChildDOB_attr::OnAfterChange(eventObj)
{
// Write code here to restore the global state. if (eventObj.IsUndoRedo)
{
// An undo or redo operation has occurred and the DOM is read-only. return; }
// A field change has occurred and the DOM is writable. Write code here to respond to the changes.
var tay=new Date()
var tdy=GetDateString(tay)
var dateObj = new Date(eventObj.Site.text.replace(/(\d\d\d\d)-(\d\d)-(\d\d)/, "$2-$3-$1"));
// var dt = dateObj.getDate(); var dt=GetDateString(dateObj)
//var dt= XDocument.DOM.selectSingleNode("my:IFSP/my:DOB").text var res =datediff1(dt,tdy,'m')
//XDocument.UI.Alert('today '+res);
var dateObj = new
Date(eventObj.Site.text.replace(/(\d\d\d\d)-(\d\d)-(\d\d)/, "$2-$3-$1"))
var dateYear = dateObj.getYear()
dateObj.setYear(dateYear + 3)
XDocument.DOM.selectSingleNode("my:myFields/my:dataFields/my:tblChild/my:ChildThirdBirthday").text = GetDateString(dateObj)
}
}
The part in BOLD is what I cannot figure out. Thanks!