Date Extensions for Javascript

DateExtensions.js

Date.prototype.clone = function () {
    return new Date(this.valueOf());
}

Date.prototype.addDays = function (days) {
    var d = this.clone();
    d.setDate(d.getDate() + days);
    return d;
}

Date.prototype.countDays = function (end) {
    var start = this.getTime();
    return Math.round((end.getTime() - this.getTime()) / (24 * 60 * 60 * 1000))
}

Date.prototype.countMonths = function (end) {
    return (end.getFullYear() * 12 + end.getMonth()) - (this.getFullYear() * 12 + this.getMonth());
}

Date.prototype.datePart = function () {
    var d = this.clone();
    d.setHours(0, 0, 0, 0);
    return d;
}

Date.prototype.weekIndexInMonth = function () {
    return Math.ceil(this.getDate() / 7) - 1;
}