Date.prototype.Format = function(fmt) 
{
    var o =
    { 
        "M+" : this.getMonth() + 1, 
        "d+" : this.getDate(), 
        "h+" : this.getHours(), 
        "m+" : this.getMinutes(), 
        "s+" : this.getSeconds(), 
        "q+" : Math.floor((this.getMonth() + 3) / 3), 
        "S" : this.getMilliseconds() 
    }; 
    if (/(y+)/.test(fmt)) 
        fmt = fmt.replace(RegExp.$1, (this.getFullYear() + "").substr(4 - RegExp.$1.length)); 
    for (var k in o) 
        if (new RegExp("(" + k + ")").test(fmt)) 
            fmt = fmt.replace(RegExp.$1, (RegExp.$1.length == 1) ? (o[k]) : (("00" + o[k]).substr(("" + o[k]).length))); 
    return fmt; 
}

Date.prototype.addDays = function(d)
{
    this.setDate(this.getDate() + d);
};
Date.prototype.subDays = function(d)
{
    this.setDate(this.getDate() - d);
};

Date.prototype.addWeeks = function(w)
{
    this.addDays(w * 7);
};

Date.prototype.addMonths= function(m)
{
    var d = this.getDate();
    this.setMonth(this.getMonth() + m);

    if (this.getDate() < d)
        this.setDate(0);
};

Date.prototype.addYears = function(y)
{
    var m = this.getMonth();
    this.setFullYear(this.getFullYear() + y);

    if (m < this.getMonth()) 
    {
        this.setDate(0);
    }
};