Date parameter in SQL Server stored procedure from ASP.NET

Date parameter in SQL Server stored procedure from ASP.NET

I've written an SQL Server stored procedure that, amongst other things,
accepts a date parameter:
CREATE PROCEDURE data.addScore
... (some parameters),
@testDate date
AS
On an ASP.NET page I have a textbox with a Calender Extender control (from
the AJAX Toolkit) that allows the user to select a date; the date is
stored in the textbox in the format DD/MM/YYYY. I have some code that is
supposed to execute the stored procedure with the parameters, using the
date value from the textbox:
cmd.Parameters["@testDate"].Value =
Convert.ToDateTime(((TextBox)li.FindControl("date")).Text);
I know I can definitely get to the textbox's value using
((TextBox)li.FindControl("date")).Text, but for the life of me I can't get
it into a format that doesn't throw up one of a few errors, mainly 'Input
string was not in the correct format'.
When I run the stored procedure directly on the server I usually bind the
parameter like @testDate = 'YYYYMMDD'.