计算年龄的函数方法

function GetAge(identityCard) {
            var len = (identityCard + "").length;
            if (len == 0) {
                return 0;
            } else {
                if ((len != 15) && (len != 18))//身份证号码只能为15位或18位其它不合法
                {
                    return 0;
                }
            }
            var strBirthday = "";
            if (len == 18)//处理18位的身份证号码从号码中得到生日和性别代码
            {
                strBirthday = identityCard.substr(6, 4) + "/" + identityCard.substr(10, 2) + "/" + identityCard.substr(12, 2);
            }
            if (len == 15) {
                strBirthday = "19" + identityCard.substr(6, 2) + "/" + identityCard.substr(8, 2) + "/" + identityCard.substr(10, 2);
            }
            //时间字符串里，必须是“/”
            var birthDate = new Date(strBirthday);
            var nowDateTime = new Date();
            var age = nowDateTime.getFullYear() - birthDate.getFullYear();
            //再考虑月、天的因素;.getMonth()获取的是从0开始的，这里进行比较，不需要加1
            if (nowDateTime.getMonth() < birthDate.getMonth() || (nowDateTime.getMonth() == birthDate.getMonth() && nowDateTime.getDate() < birthDate.getDate())) {
                age--;
            }
            return age;
        }