获取每个月递减的算法

   function lastMonth() {
            var dt = document.getElementById("d1").innerText;
            var strArr = dt.split('-');
            var year = strArr[0];
            var month = strArr[1];
            var day = strArr[2];

            //一月的时候回到上一年的12月
            if (strArr[1] == 1) {
                year = year - 1;
                month = 12;
            }
            else {
                month = month - 1;
            }
            if (month.toString().length == 1) {
                month = '0' + month;
            }
            document.getElementById("d1").innerText = year + '-' + month + '-' + day;
        }