function goPages() {
        var m = document.getElementById("m").options[document.getElementById("m").selectedIndex].value;
        var d = document.getElementById("d").options[document.getElementById("d").selectedIndex].value;
        if ((d == "30") && (m == "2")) {
            return false;
        }

        if (d == "31") {
            if ((m == "2") || (m == "4") || (m == "6") || (m == "9") || (m == "11"))
                return false;
        }
        //跳转页面;
        location.href = m + "_" + d + ".html";
    }

Done!