JavaScript date helper class

Save the following code to a .js file or add to an existing one you might have:

Date.prototype.toNoon = function () {
    this.setHours(12, 0, 0, 0);
    return this;
}
Date.prototype.dateAdd = function (interval, n) {
    if (isNaN(n = parseInt(n, 10))) {
        //Only accpets numbers
        throw "The second parameter must be a number. \n You passed: " + n;
        return null;
    }

    var dt = new Date(this);

    switch (interval.toLowerCase()) {
        case "y":
            {// year
                this.setFullYear(this.getFullYear() + n);
                break;
            }
        case "q":
            { // quarter
                this.setMonth(this.getMonth() + (n * 3));
                break;
            }
        case "m":
            { // month
                this.setMonth(this.getMonth() + n);
                break;
            }
        case "d":   // day / day of year
        case "w":
            { // weekday
                var y = this.getFullYear();
                var m = this.getMonth();
                var d = this.getDate();
                var newDate = new Date(y, m, d, 12, 0, 0);
                newDate.setDate(d + n);
                this.setTime(newDate.getTime());

                //this.setUTCDate(this.getUTCDate() + n);
                //var diff = this.getTime() - (dt.getTime() + (n*84600000));
                //this.setMilliseconds(diff);
                break;
            }
        case "ww":
            { // week of year
                this.setDate(this.getDate() + (n * 7));
                break;
            }
        case "h":
            { // hour
                this.setHours(this.getHours() + n);
                break;
            }
        case "n":
            { // minute
                this.setMinutes(this.getMinutes() + n);
                break;
            }
        case "s":
            { // second
                this.setSeconds(this.getSeconds() + n);
                break;
            }
        case "ms":
            { // milli second
                this.setMilliseconds(this.getMilliseconds() + n);
                break;
            }
        default:
            {
                //list of elegible intervals.
                throw "The first parameter must be a string from this list: \n" +
                    "y, q, m, d, w, ww, h, n, s, or ms. You passed: " + interval;
                return false;
            }
    }
    switch (interval.toLowerCase()) {
        case "d":   // day / day of year
        case "w":
            { // weekday
                break;
            }
        default:
            {
                if (this.getDate() != dt.getDate()) {
                    this.setDate(0);
                }
            }
    }
    return this;
}
Date.prototype.addDays = function (n) {
    return this.dateAdd("d", n);
}
Date.prototype.addMonths = function (n) {
    return this.dateAdd("m", n);
}
Date.prototype.firstDayOfMonth = function () {
    var y = this.getFullYear();
    var m = this.getMonth();
    this.setTime((new Date(y, m, 1, 12, 0, 0)).getTime())
    return this;
}
Date.prototype.lastDayOfMonth = function () {
    return this.firstDayOfMonth().addMonths("m", 1).addDays("d", -1);
}
Date.prototype.isBefore = function (date, orEqual) {
    return this.getTime() < date.getTime() || (orEqual && this.getTime() == date.getTime());
}
Date.prototype.isAfter = function (date, orEqual) {
    return this.getTime() > date.getTime() || (orEqual && this.getTime() == date.getTime());
}
Date.prototype.isBetween = function (a, b, orEqual) {
    return a.isBefore(this, orEqual) && this.isBefore(b, orEqual);
}
Date.prototype.range = function (date) {
    var range = {};
    if (this.isBefore(date)) {
        range["fromDate"] = new Date(this);
        range["toDate"] = new Date(date);
    } else {
        range["fromDate"] = new Date(date);
        range["toDate"] = new Date(this);
    }

    function overlaps(a, b) {
        return contains(a) || contains(b) || (a.isBefore(range["fromDate"], true) && range["toDate"].isBefore(b, true));
    }
    function contains(a) {
        return isBetween(a, range["fromDate"], range["toDate"]) || isBetween(a, range["toDate"], range["fromDate"]);
    }
    function isBetween(c, a, b) {
        return c.isBetween(a, b, true);
    }

    return {
        overlaps: overlaps,
        contains: contains,
        days: function () { return ((range["toDate"] - range["fromDate"]) / (1000 * 60 * 60 * 24)); },
        fromDate: function () { return range["fromDate"]; },
        toDate: function () { return range["toDate"]; }
    }
}
Date.prototype.monthNames = function () {
    return new Array("January", "February", "March",
        "April", "May", "June", "July", "August", "September",
        "October", "November", "December");
}
Date.prototype.monthName = function (shrt) {
    var idx = this.getMonth();
    var name = "";
    if (idx >= this.monthNames.length || idx < 0)
        return "";
    name = this.monthNames[idx];
    if (!shrt)
        return name;
    else
        return name.substr(0, 3);
}

To use, just use a Date class instance, e.g.

var dt = new Date();
dt.addDays(1);

This will add one day to the current date.