Actually, to change the default date format, use:

var task = new GlideRecord('cert_follow_on_task');
//task.addEncodedQuery('due_dateISEMPTY^active=true^stateNOT IN3,4,7^state=2^assigned_to=NULL');
task.query();
while (task.next()) {
    var date = new GlideDate();
    date.addDays(14);
    var newDate = date.getByFormat('MM/dd/yyyy');
    gs.info("New date = " + newDate + ".");
    task.due_date = newDate;

//    task.update();
    gs.info("New due_date = " + task.due_date + ".");
}

the time format in your example doesn't need changing. However, due_date is a glide DateTime value, and in my instance, the formatting is determined by the Date format set in the 'Basic Configuration' module. So the script has no effect on the formatting when viewing the due_date, but does add 14 days.