node.js 获得月份

function gM(num) {
    var new_month = 0;
    var now_date = new Date();
    var year = now_date.getFullYear();
    var month = now_date.getMonth();
    if ((month - num) < 0) {
        for (var i = 1; i < 100; i++) { //此处为最大100年以内判断
            var new_month = new_month + 12;
            if (((new_month + month) - num) > 0) {
                var new_year = year - Number(i);
                var new_month = (new_month + month) - num;
                break;
            };
        };
    } else {
        var new_year = year;
        var new_month = month - num;
    }
    return (new_year.toString() + "-" + new_month.toString() + "-" + "1" + " 00:00:00");
}