Going From UTC Miliseconds To Regular Date String?

the first thing i do is convert the date to miliseconds, add 4 weeks worth of miliseconds to it now i want to convert back, but this is where i am stuck.

function start_to_enroll(){
var miliDay = 86400000;
var startField = document.getElementById('start_date');
var enrollField = document.getElementById('enroll_date');

startField.
mili4weeks = 28 * miliDay;
miliDate = utc_date(startField.value);
total = miliDate + mili4weeks;

}
}

i just want to put total back to a date string i tried total.getUTCMonth etc. but it returns undefined.