<td style="width: 794px; height: 50px; text-align: center">
                    <asp:Label ID="label" runat="server" Height="36px" Style="z-index: 100; left: 28px;
                        position: absolute; top: 136px" Text="考勤编号：" Width="177px"></asp:Label>
                </td>
                <td style="width: 794px; height: 50px; text-align: center">
                </td>
                <td style="width: 794px; height: 50px; text-align: center">
                    <asp:Label ID="Label1" runat="server" Height="36px" Style="z-index: 100; left: 26px;
                        position: absolute; top: 297px" Text="考勤日期" Width="177px"></asp:Label>
                </td>
                <td style="width: 794px; height: 50px; text-align: center">
                    <asp:TextBox ID="T_kid" runat="server" Height="30px" Style="z-index: 100; left: 215px;
                        position: absolute; top: 137px" Width="173px"></asp:TextBox>
                    <asp:DropDownList ID="DropDownList1" runat="server" AutoPostBack="True" DataSourceID="SqlDataSource1"
                        DataTextField="lx" DataValueField="xid" Style="z-index: 102; left: 625px; position: absolute;
                        top: 143px">


protected void  Calendar1_SelectionChanged(object sender, EventArgs e)
{
 Textbox1.Text   =   Calendar1.SelectedDate.ToShortDateString();
}
    protected void  Calendar2_SelectionChanged(object sender, EventArgs e)
{
Textbox2.Text   =   Calendar2.SelectedDate.ToShortDateString();
}
}

无语啊姐姐...
就是说缺少id为Textbox1、Textbox2的Textbox控件啊 都没有你给谁赋Text啊

<td style="width: 794px; height: 50px; text-align: center">
                    <asp:Label ID="label" runat="server" Height="36px" Style="z-index: 100; left: 28px;
                        position: absolute; top: 136px" Text="考勤编号：" Width="177px"></asp:Label>
                </td>
                <td style="width: 794px; height: 50px; text-align: center">
                </td>
                <td style="width: 794px; height: 50px; text-align: center">
                    <asp:Label ID="Label1" runat="server" Height="36px" Style="z-index: 100; left: 26px;
                        position: absolute; top: 297px" Text="考勤日期" Width="177px"></asp:Label>
                </td>
                <td style="width: 794px; height: 50px; text-align: center">
                    <asp:TextBox ID="TextBox1" runat="server" Height="30px" Style="z-index: 100; left: 215px;
                        position: absolute; top: 137px" Width="173px"></asp:TextBox>
                    <asp:DropDownList ID="DropDownList1" runat="server" AutoPostBack="True" DataSourceID="SqlDataSource1"
                        DataTextField="lx" DataValueField="xid" Style="z-index: 102; left: 625px; position: absolute;
                        top: 143px">
这个应该不会在报你说的那个错了

<td style="width: 794px; height: 50px; text-align: center">
                    <asp:Label ID="label" runat="server" Height="36px" Style="z-index: 100; left: 28px;
                        position: absolute; top: 136px" Text="考勤编号：" Width="177px"></asp:Label>
                </td>
                <td style="width: 794px; height: 50px; text-align: center">
                </td>
                <td style="width: 794px; height: 50px; text-align: center">
                    <asp:Label ID="Label1" runat="server" Height="36px" Style="z-index: 100; left: 26px;
                        position: absolute; top: 297px" Text="考勤日期" Width="177px"></asp:Label>
                </td>
                <td style="width: 794px; height: 50px; text-align: center">
                    <asp:TextBox ID="TextBox1" runat="server" Height="30px" Style="z-index: 100; left: 215px;
                        position: absolute; top: 137px" Width="173px"></asp:TextBox>
<asp:TextBox ID="TextBox2" runat="server" Height="30px" Style="z-index: 100; left: 215px;
                        position: absolute; top: 137px" Width="173px"></asp:TextBox>

                    <asp:DropDownList ID="DropDownList1" runat="server" AutoPostBack="True" DataSourceID="SqlDataSource1"
                        DataTextField="lx" DataValueField="xid" Style="z-index: 102; left: 625px; position: absolute;
                        top: 143px">

<table style="z-index: 100; left: 0px; position: absolute; top: 0px">
            <tr>
                <td colspan="4" rowspan="2" style="height: 121px; text-align: center">
                    <span style="font-size: 32pt">考勤管理系统<br />
                        <span style="font-size: 11pt">&nbsp;
                            <asp:TextBox ID="TextBox1" runat="server" Height="29px" Style="z-index: 100;
                            left: 213px; position: absolute; top: 300px" Width="175px"></asp:TextBox>
                            <asp:Calendar ID="Calendar1" runat="server" Style="z-index: 101; left: 418px; position: absolute;
                                top: 298px" OnSelectionChanged="Calendar1_SelectionChanged"></asp:Calendar>
                            <asp:Calendar ID="Calendar2" runat="server" Style="z-index: 102; left: 161px; position: absolute;
                                top: 396px" OnSelectionChanged="Calendar2_SelectionChanged"></asp:Calendar>
                            &nbsp;
我这里定义了TextBox ID="TextBox1

错误解决了吗？
从你在一楼发的帖子看，确实没有TextBox1的定义，要不你把整个文件的代码发上来。