DTS error when a parameter is added to an Execute SQL Task

Question

I have an Execute SQL Task that is (hopefully) going to receive a global variable called startDate, which is set to a Date type. A shortened version of the code looks like this:

DECLARE @startDate DATETIME,
        @endDate DATETIME

SET @startDate = ?
SET @endDate = DATEADD(DAY, 6, @startDate)

INSERT INTO destinationTable
SELECT  * 
FROM    sourceTable
WHERE   startDate = @startDate 
AND     endDate = @endDate

When I replace the parameter with a date (e.g. '08-01-2012'), the statement parses correctly; however, as soon as the parameter is added back in, the statement errs.

I use DTS very rarely, so I'm basing this code entirely on Microsoft's Using Parameterized Queries. Can anyone enlighten me on how to correctly use a parameter in this situation?

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Edit: The error detail reads "Syntax error or access violation" which, considering the circumstances, I assume to read as a syntax error due to the addition of the "?".