var nowtime = new Date(); //获取当前时间
                // 只取年月日
                var nowyear = nowtime.getFullYear()
                                //月份0-11，所以+1
                var nowmonth = nowtime.getMonth() + 1
                var nowday = nowtime.getDate()

                // 添加分隔符“-”
                var seperator = "-";
                // 对月份进行处理，1-9月在前面添加一个“0”
                if (nowmonth >= 1 && nowmonth <= 9) {
                    nowmonth = "0" + nowmonth;
                }
                // 对月份进行处理，1-9号在前面添加一个“0”
                if (nowday >= 0 && nowday <= 9) {
                    nowday = "0" + nowday;
                }
                // 拼接 0000-00-00；注意格式不同计算结果也不同
                var nowdate = Date.parse(nowyear + seperator + nowmonth + seperator + nowday);
                var enddate = new Date(datetime); //定义结束时间
                var diff = enddate - nowdate
                // 向下取整
                var lefttd = Math.floor(diff / (1000 * 60 * 60 * 24))

                if (lefttd < 0) {
                    return '已过期';
                } else if (lefttd == 0) {
                    return '今天过期';
                }
                return lefttd;

倒计时天数。日期格式不同做减法运算的结果也会不同，记录一下