addWeekdays.js

// Adds n weekdays to Date d.  Weekdays are Mon-Fri.  Holidays are not accounted for.
// Adding 0 days to a weekend day will return Monday's Date.
function addWeekdays(d,n) {
    d = new Date(d.getTime());

    var day = d.getDay();

    d.setDate(
        d.getDate() + n +
        (day === 6 ? 2 : +!day) +
        (Math.floor((n - 1 + (day % 6 || 1)) / 5) * 2));

    return d;
}