Here's a new feature announcement:

Google Shopping Feed Update
We have modified the default version of our Google Shopping feed, which can be generated via the Export Products section.
This includes support for the new "availability" status of "backorder", along with the additional "availability_date" field, now required for both preorder and backorder products. The "backorder" value is populated only when using Bluepark's purchase order feature, where a specific availability date has been supplied.
"Exclude if no Google category" is now unticked by default, since a Google category is no longer required.
Please feel free to either generate a new Google Shopping feed, or modify an existing one based on these changes.

Just a little confused about how this works. For example where does it get the availability date from for backorders...? Also, what changes specifically do I need to make to an existing feed?