If you have ever used the CalendarExtender, you might have received a "Specified cast is not valid" error if populating it with null dates from the database using the SelectedDate property.
-Handling dbnull.
The following SelectedDate property correctly handles dbnulls in the data:
<cc1:CalendarExtender ID="calActivityDate" runat="server"
TargetControlID="txtActivityDate"
SelectedDate='<%# Convert.IsDBNull(Eval("activityDate")) ? null : Eval("activityDate") %>'
PopupButtonID="imgActivityDate" />
