>When did we change?

My rather elderly Shorter Oxford gives the following:

First-floor. 1663.
1. The floor next above the ground floor 1865.
2. The ground floor. Now only U.S.

I guess the given dates are relevant, but I'm not quite sure how to interpret them. Presumably 1865 is the first citation regarding the Brits having moved their first floor up one.

lusy