Sort nodes based on referenced node
You have B that references A (single value entity reference field).

Construct your view as follows:

View of content (nodes) filtered by type A

Relationship: Entity Reference: Referencing entity (A bridge to the Content entity that is referencing Content via YOUR_ENTITY_REFERENCE_FIELD.). ISSUE - are you making this required? If you do then some A's that are not referenced will not show up. If you leave it as not required then unreferenced nodes will show up in the list either first or last, depending on whether sort is ASC or DESC.

Sort Criteria: Content: Post date, and set the relationship to the relationship added.

And that is it. To test your view you might want to add the field: Post Date, and set the relationship to the relationship added. This will show the post date of the referencing node so you can easily see if it works or not.

You might also want to add a second sort criteria.