JS計算兩日期之間相差的月份

 function Amount() {
            var YueRental = document.getElementById("YueRental").value;
            var MonthRental = document.getElementById("MonthRental");
            var HalfYearRental = document.getElementById("HalfYearRental");
            var YeahRental = document.getElementById("YeahRental");
            MonthRental.value = Number(YueRental) * 3;
            HalfYearRental.value = Number(YueRental) * 6;
            YeahRental.value = Number(YueRental) * 12;

            var date1 = document.getElementById("RendStartDate").value;  //開始時間
            var date2 = document.getElementById("RendEndDate").value; //結束時間
           
            var date3 = getMonthNumber(date1, date2);
           
            var TotalAmount = document.getElementById("TotalAmount");

　　　　//保留四位小數
            TotalAmount.value =( Number(date3) * Number(YueRental)).toFixed(4);

        }
        function getMonthNumber(date1, date2) {
            //默認格式為"20030303",根據自己需要改格式和方法
            var year1 = date1.substr(0, 4);
            var year2 = date2.substr(0, 4);
            var month1 = date1.substr(5, 2);
            var month2 = date2.substr(5, 2);
            var len = (year2 - year1) * 12 + (month2 - month1);

            var day = date2.substr(8, 2) - date1.substr(8, 2);
            if (day > 0) {
                len += 1;
            }
            else if (day < 0)
            {
                len -= 1;
            }
            return len;
        }