ADDDATE based on an integer field in the record

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Using MySQL 4.1.2, I need to perform a one-time update on a table that includes
these two fields:

rec_id        [int(11)]
date_start    [date]  (currently empty)

I want to set date_start = rec_id weeks after 2004-04-04.

I think I need something like this, but MySQL rejecting all of my syntax
UPDATE `mytable` SET `date_start` = ADDDATE('2007-02-04', INTERVAL `rec_id`

Any and all advice will be greatly appreciated!

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