Make use of the combination of a <asp:TextBox>
and <asp:CalendarExtender>
control that targets a textbox where the selected date will be added
<asp:TextBox ID="txtBirthdate" runat="server"
CssClass="contentfield" Height="16px" MaxLength="12"
width="150px" Wrap="False"></asp:TextBox>
<asp:CalendarExtender ID="CalendarExtender1" runat="server"
Format="dd MMM yyyy" PopupButtonID="imgSummaryCashFlowDate"
TargetControlID="txtBirthdate">
</asp:CalendarExtender>
This will work exactly the same as a normal textbox in your gridview and you will be able to pull the date from the textbox exactly the same manner as you have done orignally.