How do I add 6 or 12 weeks to date selected in a date field

I have a datefield, holddate, that I want to add 6 or 12 weeks to, based on a radiobutton click, and display the new date in another datefield, restartdate.......

the holddate defaults to Date(), unless changed by the user.... formatted "MM-DD-YYYY"

my code looks like:

<eventactivity="click" name="event__click">

<scriptcontentType="application/x-javascript">

var

dtToday = this.resolveNode("HoldDate").rawValue;

app.alert(dtToday);

var

intWkDate=Date2Num(form1.LabCoatOrder.HoldDate);

app.alert(intWkDate);

</script>

app.alert(dtToday); displays 2015-06-19

app.alert(intWkDate); this doesn't display..... haven't even tried adding 42 or 84 days yet!!!!! trying to determine the days from epoch so I can add the additional days.