C# - How To Compare The Difference Of Date In Gridveiw Textbox

i have designed a gridview in that one of my template field will be like

<asp:TemplateField>
<ItemTemplate>
<asp:Label ID="lblTrialPeriodEnds" runat="server" Text='<%# Eval("trialPeriodEnds","{0:dd/MM/yyyy}") %>'
Width="61px"></asp:Label>
</ItemTemplate>
<HeaderTemplate>
TrialPeriodEnds
</HeaderTemplate>
<EditItemTemplate>
<asp:TextBox Id="txtRenew" runat="server" Text='<%# Eval("trialPeriodEnds","{0:dd/MM/yyyy}") %>'
Width="61px" />
</EditItemTemplate>
</asp:TemplateField>

Now what i need is i will have a pop up calendar after the text box when i click on edit . If i select a date greater than the date existed in the text box i would like to set my Database field to Yes..