This has to be done completely in code as there is no way to do a date diff
through the UI.

Here was a suggestion listed a while ago:

Date diff:

Mathmatical operations involving two time values don't make much sense
IMHO.  Usually it's a time +/- an offset like X hours or something.
But, here goes:

var dom = XDocument.DOM;
var t1 = dom.selectSingleNode("my:myFields/my:time1").nodeTypedValue;
var t2 = dom.selectSingleNode("my:myFields/my:time2").nodeTypedValue;
var t = null;

if (t1.match(/T(\d\d):(\d\d):(\d\d)/))
  t1 = Number(RegExp.$1)*3600 + Number(RegExp.$2)*60 +
Number(RegExp.$3);
else
  XDocument.UI.Alert("Time 1 is invalid.");

if (t2.match(/T(\d\d):(\d\d):(\d\d)/))
  t2 = Number(RegExp.$1)*3600 + Number(RegExp.$2)*60 +
Number(RegExp.$3);
else
  XDocument.UI.Alert("Time 2 is invalid.");

t = t1 + t2;

var h = Math.floor(t / 3600);
var m = Math.floor(t % 3600 / 60);
var s = t % 60;

t = String(h + ":" + m + ":" + s).replace(/\b(\d)\b/g, "0$1");

XDocument.UI.Alert(t);

--josh bertsch