The Brooklands visit was on July 20th. The Timeline page is currently showing their earliest possible wedding date as (Tuesday) July 21st. Surely this must be wrong. Didn't they agree to a quick wedding "this Saturday", making July 25th the earliest possible date ?
Also, when Henry paid the surprise visit to Downton, I think Robert said to him "We've not seen you since Brooklands", which suggests that some time had elapsed since then. This makes July 25th also seem far too early for the wedding date.