I imported my scrobbles from last.fm and the data seems to be about 1 year off. My first scrobbles to last.fm were in January 2006, but the first scrobbles that get imported to listenbrainz are in february 2005, which is not possible (I only created my account in January 2006).

https://listenbrainz.org/user/csaavedra/

https://www.last.fm/user/csaavedra