When you are adding products to your Google Shopping feed that are on "backorder" Google requires you to also add an availability date to your product feed. Since WooCommerce doesn't come out-of-the-box with such an attribute or field where you can insert your availability dates we have added 8 attributes to our plugin you can use to dynamically set an availability date so those end up in your product feed. Availability date + 1 week Availability date + 2 ...