疑惑:jS与下拉框的冲突


<tr>
                    <td style="width: 80px; height: 32px;" class="Login2">
                        企业法人：<span style="color: #ff0000">*</span></td>
                    <td style="width: 215px; height: 32px;" class="Login2">
                        <asp:TextBox ID="txtCorp" runat="server"></asp:TextBox>
                    </td>
                    
                    <td style="width: 88px; height: 32px" class="Login2">
                        注册日期：
                    </td>
                    <td style="width: 253px; height: 32px;" class="Login2">
                        <input type="text" id="txtBuyDate" runat="server" readonly="readOnly" />
                        <a href="javascript:;" onclick='showCalendar(this, document.getElementById("ctl00_maincontent_txtBuyDate"), "yyyy-mmmm-dd","en",0,-1,-1)'>
                            <img src="../images/icon-calendar.gif" style="width: 20px; height: 19px; border-top-style: none;
                                border-right-style: none; border-left-style: none; border-bottom-style: none;" /></a>
                    </td>
                </tr>
                <tr>
                   <td style="width: 80px; height: 32px" class="Login2">
                        注册类型：<span style="color: #ff0000">*</span></td>
                    <td style="width: 215px; height: 32px;" class="Login2">
                        &nbsp;<asp:DropDownList ID="DDLType" runat="server" Style="position: relative"
                            Width="149px">
                            <asp:ListItem>农机合作社</asp:ListItem>
                            <asp:ListItem>农机大户</asp:ListItem>
                            <asp:ListItem>农机作业公司</asp:ListItem>
                            <asp:ListItem>农机协会</asp:ListItem>
                        </asp:DropDownList></td> 
                    <td style="width: 88px; height: 32px;" class="Login2">
                        注册资金： <span style="color: #ff0000">*</span></td>
                    <td style="width: 253px; height: 32px;" class="Login2">
                        <asp:TextBox ID="txtRegMoney" runat="server"></asp:TextBox>
                    </td>
                </tr>代码如上
环境:vs2005+xp sp3(使用了母版页)
问题描述:
1. JS日历能够正常弹出,但每次弹出日历时会使DropDownlist隐藏,选择日期后 则恢复
2. JS日历根本不弹出,出现缺少对象错误.此时DropDownlist不会隐藏,如果把页面上的DropDownlist去掉后
   JS日历则弹出正常.