the datetime variable value not null it always minvalue

Question

i have aform that filter data usig four controls..we can filter using one or more control....so the controls may be null and this is the problem 
this is my aspx page code 

   <asp:EntityDataSource ID="BuyEntityDataSource" runat="server" ConnectionString="name=StockyEntities"

                    DefaultContainerName="StockyEntities" EnableFlattening="False" EntitySetName="Buys"

                    Include="Supplier" Where="(it.OrganizationID=@OrganizationID) AND (it.BuyDate >= @DateFrom or @DateTo IS NULL ) AND ( it.BuyDate <= @DateTo or @DateTo IS NULL ) AND (it.InvoiceNumber=@BuyID or @BuyID IS NULL) AND (it.Supplier.SupplierName=@Supplier or @Supplier IS NULL)"

                    EntityTypeFilter="Buy">

                    <WhereParameters>

                        <asp:Parameter Type="Int64" Name="BuyID" />

                        <asp:Parameter Type="String" Name="Supplier" />

                        <asp:Parameter Type="DateTime" Name="DateFrom" />

                        <asp:Parameter Type="DateTime" Name="DateTo" />

                        <asp:SessionParameter DefaultValue="1" Name="OrganizationID" SessionField="OrganizationID"

                            Type="Int32" />

                    </WhereParameters>

                </asp:EntityDataSource>

and this is the code in aspx.cs file code ... so what can i do ..... please help me quicky ...

  DateTime fromDate; DateTime.TryParseExact(FromDateTextBox.Text, "dd/MM/yyyy", null, System.Globalization.DateTimeStyles.None, out fromDate);  //if the control is null it retruns Minvalue (1/1/0001 12:00:00 AM)

            DateTime toDate; DateTime.TryParseExact(ToDateTextBox.Text, "dd/MM/yyyy", null, System.Globalization.DateTimeStyles.None, out toDate);  //if the control is null it retruns Minvalue (1/1/0001 12:00:00 AM)

            string BuyId = BuyIDTextBox.Text;

            string SupplierName = SupplierTextBox.Text;

            string fromDateString = fromDate.ToString();//to current culture format

            string toDateString = toDate.ToString();//to current culture format

            BuyEntityDataSource.WhereParameters["DateTo"].DefaultValue = toDateString;

            BuyEntityDataSource.WhereParameters["DateFrom"].DefaultValue = fromDateString;

            BuyEntityDataSource.WhereParameters["Supplier"].DefaultValue = SupplierName;

            BuyEntityDataSource.WhereParameters["BuyID"].DefaultValue = BuyId;