js时间日期的三级联动

<div style="width:70px;float:left;">生日</div>
            <asp:DropDownList ID="DropDownList1" runat="server"></asp:DropDownList>年
            <asp:DropDownList ID="DropDownList2" runat="server"></asp:DropDownList>月
            <asp:DropDownList ID="DropDownList3" runat="server"></asp:DropDownList>日

<script>
　　//年份变动的事件
    document.getElementById("DropDownList1").onchange = function () {
        var mon = document.getElementById("DropDownList2");
        var day = document.getElementById("DropDownList3");
        if (mon.value == "2")
        {
            if (this.value % 4 == 0 && this.value % 100 != 0 || this.value % 400 == 0) {
                day.options.length = 0;
                for (var i = 1; i < 30; i++) {
                    var op = document.createElement("option");
                    op.value = i;
                    op.innerHTML = i;
                    day.appendChild(op);
                }
            }
            else {
                day.options.length = 0;
                for (var i = 1; i < 29; i++) {
                    var op = document.createElement("option");
                    op.value = i;
                    op.innerHTML = i;
                    day.appendChild(op);
                }
            }
        }
        else if (mon.value == "1" || mon.value == "3" || mon.value == "5" || mon.value == "7" || mon.value == "8" || mon.value == "10" || mon.value == "12") {
            day.options.length = 0;
            for (var i = 1; i <= 31; i++) {
                var op = document.createElement("option");
                op.value = i;
                op.innerHTML = i;
                day.appendChild(op);
            }
        }
        else {
            day.options.length = 0;
            for (var i = 1; i <= 30; i++) {
                var op = document.createElement("option");
                op.value = i;
                op.innerHTML = i;
                day.appendChild(op);
            }
        }
    }
　　//月变动的事件
    document.getElementById("DropDownList2").onchange = function () {
        var day = document.getElementById("DropDownList3");
        var year = document.getElementById("DropDownList1");
        if (this.value == "2")
        {
            if (year.value % 4 == 0 && year.value % 100 != 0 || year.value % 400 == 0) {
                day.options.length = 0;
                for (var i = 1; i < 30; i++) {
                    var op = document.createElement("option");
                    op.value = i;
                    op.innerHTML = i;
                    day.appendChild(op);
                }
            }
            else {
                day.options.length = 0;
                for (var i = 1; i < 29; i++) {
                    var op = document.createElement("option");
                    op.value = i;
                    op.innerHTML = i;
                    day.appendChild(op);
                }
            }
        }
        else if (this.value == "1" || this.value == "3" || this.value == "5" || this.value == "7" || this.value == "8" || this.value == "10" || this.value == "12") {
            day.options.length = 0;
            for (var i = 1; i <= 31; i++) {
                var op = document.createElement("option");
                op.value = i;
                op.innerHTML = i;
                day.appendChild(op);
            }
        }
        else {
            day.options.length = 0;
            for (var i = 1; i <= 30; i++) {
                var op = document.createElement("option");
                op.value = i;
                op.innerHTML = i;
                day.appendChild(op);
            }
        }
    }

   


</script>

if (!IsPostBack)
        {
            for (int i = DateTime.Now.Year; i >= 1900; i--)
            {
                ListItem li = new ListItem(i.ToString(), i.ToString());
                DropDownList1.Items.Add(li);
            }
            for (int i = 1; i < 13; i++)
            {
                DropDownList2.Items.Add(new ListItem(i.ToString(), i.ToString()));
            }
            for (int i = 1; i < 31; i++)
            {
                DropDownList3.Items.Add(new ListItem(i.ToString(), i.ToString()));
            }
        
        
        }