You can't pass in a function call as an argument to your stored procedure. Instead use an intermediate variable:

DECLARE @tmp DATETIME
SET @tmp = GETDATE()

EXEC DisplayDate @tmp;

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'

the solution I found was to declare temp variables before executing and pass these into the execution line e.g.

Declare @ddate date

set ddate = getdate()

EXEC DisplayDate @ddate;