A combo box don't need code behide. The important is the combo box is link to the subform.
This is very strange, yah, I have the same problem with the format date, but if you create the combo box by wizard. It gives a good result. See the attachment. Also, I have created a new column "IDMonthYear" into the Customers table and I create a new column into MonthYear table. Take a look on these tables. Hope this help.
Le