How can I rank my retrieved documents

Depends on what you mean by rank?

We can sort and filter by date using node-postprocessors

https://gpt-index.readthedocs.io/en/latest/examples/node_postprocessor/RecencyPostprocessorDemo.html

https://gpt-index.readthedocs.io/en/latest/examples/node_postprocessor/TimeWeightedPostprocessorDemo.html#define-recency-postprocessors

How do I specify the name of the weaviate property to the fixedrecencypostprocessor? Does it only work if the field is called “date”? Also I’m not passing in documents, I’m using an existing weaviatev vector store

That's fine, the node post processor works on nodes once they are retrieved

There is a date_key kwarg in the init that you can set to match whichever metadat field your date is stored in