As Mitch Wheat mentioned you can't pass a function.
If in your case you should pass in a precalculated value or GETDATE() - you can use default value. For example, modify your stored procedure:
ALTER PROC DisplayDate
(
@DateVar DATETIME = NULL
) AS
BEGIN
set @DateVar=ISNULL(@DateVar,GETDATE())
--the SP stuff here
SELECT @DateVar
END
GO
And then try:
EXEC DisplayDate '2013-02-01 00:00:00.000'
EXEC DisplayDate
Remark: Here I supposed that NULL value is not in use for this parameter. If it is not your case - you can use another unused value, for example '1900-01-01 00:00:00.000'