【javascript】日付増減日数

//beginDate          ，days      ，      ，    ：YYYY-MM-DD
    function getDay(beginDate, days) {
        var beginDate = beginDate.split("-");
        var nDate = new Date(beginDate[1] + '-' + beginDate[2] + '-' + beginDate[0]); //   MM-DD-YYYY    
        var millSeconds = Math.abs(nDate) + (days * 24 * 60 * 60 * 1000);
        var rDate = new Date(millSeconds);
        var year = rDate.getFullYear();
        var month = rDate.getMonth() + 1;
        if (month < 10) month = "0" + month;
        var date = rDate.getDate();
        if (date < 10) date = "0" + date;
        return (year + "-" + month + "-" + date);
    }