DataSource Controls :: Filter - Select Date In Ddl And Use Selectedvalue In Gridview SelectCommand

I've got a gridview which works fine. I use dropdownlists which are filled with distinct values of the columns. So when I select an item in a drompdownlist the gridview refreshes based on the selected value. When I have a column which uses string values, I can start the filter with a default % value to use in my select paramaters. But when I have a date column, I can't use that. I use a datasource for the gridview and I prefer to leave codebehind out if this is not needed. Now I use >= in my gridview select to show all rows. but I want to use only = to make the filter work correct when I select a date in the ddl and use the selectedvalue in the gridview selectCommand

<asp:DropDownList ID="ddlUpdateDatum" DataSourceID="dsPopulateUpdateDatum"
AutoPostBack="true" DataValueField="UPDATEDATE" runat="server"
CssClass="selectinput" AppendDataBoundItems="true">
<asp:ListItem Text="All" Value="01/01/1900"></asp:ListItem>
</asp:DropDownList>
<asp:SqlDataSource ID="dsPopulateUpdateDatum" runat="server"
ConnectionString="<%&#36; ConnectionStrings:GAMSYSTEMSqlServer %>"
SelectCommand="SELECT DISTINCT(UPDATEDATE) FROM [messages] ORDER BY UPDATEDATE ASC ">
</asp:SqlDataSource>
And I use this in the gridview datasource:
<asp:SqlDataSource
runat="server"
ID="dsGridview"
ConnectionString="<%&#36; ConnectionStrings:DBSqlServer %>"
SelectCommand = "SELECT * FROM [messages] WHERE AND UpdateDatum >= @UpdateDatePar ORDER BY UpdateDatumDESC"
>
<SelectParameters>
<asp:ControlParameter Name="UpdateDatePar" ControlID="ddlUpdateDate" PropertyName="SelectedValue" Type="string" />
</SelectParameters>