<div>
        <select name="YYYY" onchange="YYYYMM(this.value)">
            <option value=""> Please select a   years </option>
        </select>
        <select name="MM" onchange="MMDD(this.value)">
            <option value=""> choose   month </option>
        </select>
        <select name="DD">
            <option value=""> choose   day </option>
        </select>
    </div>
    <script language="JavaScript">
        function window.onload() {
            strYYYY = document.form1.YYYY.outerHTML;
            strMM = document.form1.MM.outerHTML;
            strDD = document.form1.DD.outerHTML;
            MonHead = [31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31];
            //First give the contents of the year drop - down box
            var y = new Date().getFullYear();
            var str = strYYYY.substring(0, strYYYY.length - 9);
            for (var i = (y - 30); i < (y + 30); i++) //This year shall prevail, the first 30 years and the last 30 years
            {
                str += "<option value='" + i + "'> " + i + "  years " + "</option>rn";
            }
            document.form1.YYYY.outerHTML = str + "</select>";
            //The drop-down box for the assigned month
            var str = strMM.substring(0, strMM.length - 9);
            for (var i = 1; i < 13; i++) {
                str += "<option value='" + i + "'> " + i + "  month " + "</option>rn";
            }
            document.form1.MM.outerHTML = str + "</select>";
            document.form1.YYYY.value = y;
            document.form1.MM.value = new Date().getMonth() + 1;
            var n = MonHead[new Date().getMonth()];
            if (new Date().getMonth() == 1 && IsPinYear(YYYYvalue)) n++;
            writeDay(n); //Assign date drop-down box
            document.form1.DD.value = new Date().getDate();
        }
        function YYYYMM(str) //When the year changes, the date changes (mainly to judge the run-flat year)
        {
            var MMvalue = document.form1.MM.options[document.form1.MM.selectedIndex].value;
            if (MMvalue == "") { DD.outerHTML = strDD; return; }
            var n = MonHead[MMvalue - 1];
            if (MMvalue == 2 && IsPinYear(str)) n++;
            writeDay(n)
        }
        function MMDD(str) //Date linkage when month changes
        {
            var YYYYvalue = document.form1.YYYY.options[document.form1.YYYY.selectedIndex].value;
            if (str == "") { DD.outerHTML = strDD; return; }
            var n = MonHead[str - 1];
            if (str == 2 && IsPinYear(YYYYvalue)) n++;
            writeDay(n)
        }
        function writeDay(n) //A drop-down box that conditionally writes the date
        {
            var s = strDD.substring(0, strDD.length - 9);
            for (var i = 1; i < (n + 1); i++)
                s += "<option value='" + i + "'> " + i + "  day " + "</option>rn";
            document.form1.DD.outerHTML = s + "</select>";
        }
        function IsPinYear(year)//Judge whether a leap year
        {
            return (0 == year % 4 && (year % 100 != 0 || year % 400 == 0))
        }