Basic date add function in JavaScript.

Date.add.js

/*
Copyright (c) 2013 Ben Lesh
[email protected]
MIT License
*/

;(function () {
    var proto = Date.prototype;
    function adder(value, getter, setter, toAdd) {
        var d = new Date(value),
          cv = getter.call(d);
        setter.call(d, cv + toAdd);
        return +d;
    }

    proto.addDays = function (days) {
        return new Date(adder(this,
            proto.getDate,
            proto.setDate,
            days));
    };
    
    proto.addHours = function (hours) {
        return new Date(adder(this,
            proto.getHours,
            proto.setHours,
            hours));
    };

    proto.addMinutes = function (minutes) {
        return new Date(adder(this,
            proto.getMinutes,
            proto.setMinutes,
            minutes));
    };
    
    proto.addSeconds = function (seconds) {
        return new Date(adder(this,
            proto.getSeconds,
            proto.setSeconds,
            seconds));
    };
    
     proto.addMilliseconds = function (ms) {
        return new Date(adder(this,
            proto.getMilliseconds,
            proto.setMilliseconds,
            ms));
    };
    
    proto.addYears = function (years) {
        return new Date(adder(this,
            proto.getFullYear,
            proto.setFullYear,
            years));
    };
    
    proto.addMonths  = function (months) {
        return new Date(adder(this,
            proto.getMonth,
            proto.setMonth,
            months));
    };
})();