It is once per calendar month, so you can dine July 31str and Aug 1 and both will count for miles.
Has anyone had this experience? You get a new directory and it has a through date. A new restaurant is listed, so you go to it. You then don't get miles because on the date that you went (after you saw it in the new book) it wasn't yet in the program. Because of this we missed the 500 bonus miles with AA elite plus the miles for a $16 meal. If we had known we would have gone elsewhere for our third restaurant of the month. If anyone has successfully resolved this kind of problem I would appreciate it if you would post the details. I'm planning on calling AA customer service tomorrow.

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DtG