MyDataSource.SelectParameters["startDate"].DefaultValue =
                                     fromDate.SelectedDate.Date.ToString();

<asp:SqlDataSource ID="MyDataSource" runat="server" ConnectionString="<%$ ConnectionStrings:sampledb %>" SelectCommand="Recording_Select" SelectCommandType="StoredProcedure" OnSelecting="MyDataSource_Selecting">
    <SelectParameters>
        <asp:ControlParameter ControlID="respondentID" DefaultValue="%" Name="RespondentID" PropertyName="Text" Type="String" />
        <asp:SessionParameter Name="projectCode" SessionField="proj" Type="String" />
        <asp:ControlParameter ControlID="interviewerList" Name="InterviewerID" PropertyName="SelectedValue" Type="String" />
        <asp:ControlParameter ControlID="dispCodeList" Name="dispCode" PropertyName="SelectedValue" Type="Int32" />
        <asp:ControlParameter ControlID="fromDate" DefaultValue="1/1/1900" Name="startDate" PropertyName="SelectedValue" Type="DateTime" />
        <asp:ControlParameter ControlID="toDate" DefaultValue="1/1/1900" Name="endDate" PropertyName="SelectedValue" Type="DateTime" />
    </SelectParameters>
</asp:SqlDataSource>

DateTime DotNetDateTime = new DateTime(2009, 5, 4);
System.Data.SqlTypes.SqlDateTime sqlDateTime = new System.Data.SqlTypes.SqlDateTime(DotNetDateTime);

System.Data.SqlTypes.SqlDateTime sqlDateTime = new System.Data.SqlTypes.SqlDateTime(2009, 5, 4);

MyDataSource.SelectParameters["startDate"].DefaultValue =
                                     fromDate.SelectedDate.Date.ToString("yyyyMMdd HH:mm:ss");

MyDataSource.SelectParameters["startDate"].DefaultValue = fromDate.SelectedDate.Date.ToString("yyyy-MM-dd HH:mm:ss")