See Here:
WinJS.UI.DatePicker
There you can see you can attach a onchange listener to it, to see when it has changed, and you can also read the current value with .current i.e:
var cal = document.querySelector("calendar");
var calControl = cal.winControl;
var textbox = document.querySelector("textbox");
calControl.onchange = function () {
//format this how ever you want
textbox.value = calControl.current.toString()
}
