To scroll all documents, you should pass the X-VTEX-MD-TOKEN header value as the _token query parameter in subsequent requests to retrieve the next page.
Example initial request:
GET /api/dataentities/{data_entity}/scroll?_size={page_size}
Subsequent requests to retrieve additional pages:
GET /api/dataentities/{data_entity}/scroll?_token={X-VTEX-MD-TOKEN}
The REST-Content-Total response header informs the total number of documents that match your query, allowing you to calculate the number of pages available based on the _size parameter.
As for date filters, the format you are trying should work, or you can try a date range like this example: /dataentities/CL/search?_where=createdIn between 2001-01-01 AND 2016-01-01