DataSource Controls :: Filter Parameters/Search Form

I've got a gridview that I need to perform filters on. All works great until I try and add a date range..

Below is the code. Basically I have 5 dropdown lists which all filter as expected giving them the option to use each one or just the ones they need.

The problem is they want to also have the option of filtering by date i.e date range or greater than or less than or equal to etc..

As soon as I add the date parameter it stops the other filters working. I'm guessing because I have a null value in the date field (textbox) if they dont use the option??

Does anyone have a sample on how to filter by dropdown lists and dates keeping in mind they wont use all the filter options?

<asp:Content ID="Content1" ContentPlaceHolderID="HeadContent" runat="server">
<style type="text/css">
.style2
{
width: 100%;
}
.style3
{
width: 137px;
}
</style>
</asp:Content>
<asp:Content ID="Content2" ContentPlaceHolderID="MainContent" runat="server">
</br>
<table>
<tr>
<td>
Person Responsible</td>
<td>
<asp:DropDownList ID="DDLPersonResponsible" runat="server"
DataSourceID="SqlDataSource2" DataTextField="AssignedTo"
DataValueField="AssignedTo" AppendDataBoundItems="True"
AutoPostBack="True" Height="22px" Width="200px">
<asp:ListItem Text="All" Value="%"></asp:ListItem>
</asp:DropDownList>
</td>
</tr>
<tr>
<td>
Location</td>
<td>
<asp:DropDownList ID="DDlLocation" runat="server" Height="22px" Width="200px"
DataSourceID="SqlDataSource3" DataTextField="location"
DataValueField="location" AppendDataBoundItems="True"
AutoPostBack="True">
<asp:ListItem Text="All" Value="%"></asp:ListItem>
</asp:DropDownList>
<asp:SqlDataSource ID="SqlDataSource3" runat="server"
ConnectionString="<%$ ConnectionStrings:ASPNETDBConnectionString1 %>"
SelectCommand="usp_Locations" SelectCommandType="StoredProcedure">
</asp:SqlDataSource>
</td>
</tr>
<tr>
<td>
Activity/Product/Service</td>
<td>
<asp:DropDownList ID="DDlActivity" runat="server" DataSourceID="SqlDataSource4"
DataTextField="Activity" AppendDataBoundItems="True"
AutoPostBack="True" Height="22px" Width="200px">
<asp:ListItem Text="All" Value="%"></asp:ListItem>
</asp:DropDownList>
<asp:SqlDataSource ID="SqlDataSource4" runat="server"
ConnectionString="<%$ ConnectionStrings:ASPNETDBConnectionString1 %>"
SelectCommand="usp_Activity" SelectCommandType="StoredProcedure">
</asp:SqlDataSource>
</td>
</tr>
<tr>
<td>
Risk Ranking</td>
<td>
<asp:DropDownList ID="DDlRiskRanking" runat="server" AppendDataBoundItems="True"
AutoPostBack="True" >
<asp:ListItem Value="%">All</asp:ListItem>
<asp:ListItem>1</asp:ListItem>
<asp:ListItem>2</asp:ListItem>
<asp:ListItem>3</asp:ListItem>
<asp:ListItem>4</asp:ListItem>
</asp:DropDownList>
</td>
</tr>
<tr>
<td>
Reference</td>
<td>
<asp:DropDownList ID="DDLReference" runat="server" DataSourceID="SqlDataSource5"
DataTextField="Reference" AppendDataBoundItems="True"
AutoPostBack="True" Height="22px" Width="200px">
<asp:ListItem Text="All" Value="%"></asp:ListItem>
</asp:DropDownList>
<asp:SqlDataSource ID="SqlDataSource5" runat="server"
ConnectionString="<%$ ConnectionStrings:ASPNETDBConnectionString1 %>"
SelectCommand="usp_SelectReference" SelectCommandType="StoredProcedure">
</asp:SqlDataSource>
</td>
</tr>
<tr>
<td>
Target Date</td>
<td style="margin-left: 40px">Greater than
<asp:TextBox ID="txtstartTargetDate" runat="server" AutoPostBack="True"></asp:TextBox> Less Than
<asp:TextBox ID="txtendTargetDate" runat="server" AutoPostBack="True"></asp:TextBox>
</td>
</tr>
</table>
<asp:SqlDataSource ID="SqlDataSource2" runat="server"
ConnectionString="<%$ ConnectionStrings:ASPNETDBConnectionString1 %>"
SelectCommand="usp_AssignOwner" SelectCommandType="StoredProcedure">
</asp:SqlDataSource>
<br />
<asp:GridView ID="GridView1" runat="server" AllowSorting="True"
AutoGenerateColumns="False" DataKeyNames="IssueId"
DataSourceID="SqlDataSource1" Width="803px" CellPadding="4"
ForeColor="#333333"
EmptyDataText="There are no matching records to display.">
<AlternatingRowStyle BackColor="White" />
<Columns>
<asp:BoundField DataField="IssueId" HeaderText="IssueId" InsertVisible="False"
ReadOnly="True" SortExpression="IssueId" />
<asp:BoundField DataField="BriefDescription" HeaderText="BriefDescription"
SortExpression="BriefDescription" />
<asp:BoundField DataField="DateEntered" HeaderText="DateEntered"
SortExpression="DateEntered" />
<asp:BoundField DataField="EnteredBy" HeaderText="EnteredBy"
SortExpression="EnteredBy" />
<asp:BoundField DataField="Likelihood" HeaderText="Likelihood"
SortExpression="Likelihood" />
<asp:BoundField DataField="Consequence" HeaderText="Consequence"
SortExpression="Consequence" />
<asp:BoundField DataField="Location" HeaderText="Location"
SortExpression="Location" />
<asp:BoundField DataField="Activity" HeaderText="ActivityProductService"
SortExpression="Activity" />
<asp:BoundField DataField="Scope" HeaderText="Scope"
SortExpression="Scope" />
<asp:BoundField DataField="Aspect"
HeaderText="Aspect" SortExpression="Aspect" />
<asp:BoundField DataField="AspectRevised" HeaderText="AspectRevised"
SortExpression="AspectRevised" />
<asp:BoundField DataField="Impact" HeaderText="Impact"
SortExpression="Impact" />
<asp:BoundField DataField="Objective" HeaderText="Objective"
SortExpression="Objective" />
<asp:BoundField DataField="KeyAction" HeaderText="KeyAction"
SortExpression="KeyAction" />
<asp:BoundField DataField="TargetDate" HeaderText="TargetDate"
SortExpression="TargetDate" />
<asp:BoundField DataField="PersonResponsible" HeaderText="PersonResponsible"
SortExpression="PersonResponsible" />
<asp:BoundField DataField="PercentageCompleted"
HeaderText="PercentageCompleted" SortExpression="PercentageCompleted" />
<asp:BoundField DataField="Reference" HeaderText="Reference"
SortExpression="Reference" />
<asp:BoundField DataField="Comments" HeaderText="Comments"
SortExpression="Comments" />
<asp:BoundField DataField="Status" HeaderText="Status"
SortExpression="Status" />
<asp:BoundField DataField="AssignedTo" HeaderText="AssignedTo"
SortExpression="AssignedTo" />
<asp:BoundField DataField="RiskRanking" HeaderText="Risk Ranking" />
</Columns>
<EditRowStyle BackColor="#2461BF" />
<FooterStyle BackColor="#507CD1" Font-Bold="True" ForeColor="White" />
<HeaderStyle BackColor="#507CD1" Font-Bold="True" ForeColor="White" />
<PagerStyle BackColor="#2461BF" ForeColor="White" HorizontalAlign="Center" />
<RowStyle BackColor="#EFF3FB" />
<SelectedRowStyle BackColor="#D1DDF1" Font-Bold="True" ForeColor="#333333" />
<SortedAscendingCellStyle BackColor="#F5F7FB" />
<SortedAscendingHeaderStyle BackColor="#6D95E1" />
<SortedDescendingCellStyle BackColor="#E9EBEF" />
<SortedDescendingHeaderStyle BackColor="#4870BE" />
</asp:GridView>
<br />
<asp:SqlDataSource ID="SqlDataSource1" runat="server"
ConnectionString="<%$ ConnectionStrings:ASPNETDBConnectionString1 %>"
SelectCommand="usp_AllIssues" SelectCommandType="StoredProcedure" FilterExpression="assignedto like '{0}' and location like '{1}' and activity like '{2}' and riskranking like '{3}' and reference like '{4}' and targetdate > '{5}'">
<FilterParameters>
<asp:ControlParameter Name="assignedto" ControlID="ddlpersonresponsible"
PropertyName="SelectedValue" />
<asp:ControlParameter Name="location" ControlID="ddllocation"
PropertyName="SelectedValue" />
<asp:ControlParameter Name="activity" ControlID="ddlactivity"
PropertyName="SelectedValue" />
<asp:ControlParameter Name="riskranking" ControlID="ddlriskranking"
PropertyName="SelectedValue" />
<asp:ControlParameter Name="Reference" ControlID="ddlreference"
PropertyName="SelectedValue" />
<asp:ControlParameter Name="startdate" ControlID="txtStartTargetDate"
PropertyName="text" />
</FilterParameters>
</asp:SqlDataSource>
</asp:Content>