Grid View Date issue - easy fix I am sure

I have a dropdown list (ddlLastUpdate) that I am using to filter using date.  Everything works great except when I want to see all of the values.  Normally I select the blank spot in the dropdown list and get all records for the other filters but in this case I get a "Cannot perform '=' operation on System.DateTime and System.String." error

<asp:sqldatasource runat="server" id="MySource"
   connectionstring="<%$ ConnectionStrings:CompEvents_DSNConnection %>" ProviderName="<%$ ConnectionStrings:CompEvents_DSNConnection.ProviderName %>"
   SelectCommand="SELECT * FROM [Events]"
   DeleteCommand="DELETE FROM [Events] WHERE [EventID] = ?"
   UpdateCommand="UPDATE Events SET DivStatus = ?, LastUpdate = ?, LastUpdateBy = ? WHERE EventID = ? "  
  filterexpression="DMA Like '{0}' AND DivStatus Like '{1}' AND Division Like '{2}' AND Retailer Like '{3}' AND Action Like '{4}' and LastUpdate = '{5}'  AND ARStatus Like '{6}'"
   >
 
    <UpdateParameters>
          <asp:Parameter Name="DivStatus" Type="String" />
          <asp:SessionParameter Name="gsLastUpdDateTime" SessionField="SysDate" Type=dateTime/>
          <asp:SessionParameter Name="gsUserName" SessionField="UserName" type=string/>
          <asp:Parameter Name="EventID" Type="Int32" />
    </UpdateParameters>

    <filterparameters>
        <asp:ControlParameter ControlID="ddlDMA" DefaultValue="*" Name="dma" PropertyName="SelectedValue" />
        <asp:ControlParameter ControlID="ddlDivStatus" DefaultValue="*" Name="DivStatus" PropertyName="SelectedValue" />
        <asp:ControlParameter ControlID="ddlDivision" DefaultValue="*" Name="Division" PropertyName="SelectedValue" />
        <asp:ControlParameter ControlID="ddlRetailer" DefaultValue="*" Name="Retailer" PropertyName="SelectedValue" />
        <asp:ControlParameter ControlID="ddlAction" DefaultValue="*" Name="Action" PropertyName="SelectedValue" />
        <asp:ControlParameter ControlID="ddlLastUpdate" DefaultValue="*" Name="LastUpdate" PropertyName="SelectedValue"/>
        <asp:ControlParameter ControlID="ddlARStatus" DefaultValue="*" Name="ARStatus" PropertyName="SelectedValue" />
   </filterparameters>