Funnily enough, being a British based company, BA use GMT for the release of seats. When seats aRe released at midnight they should be available both online and if calling up at the same time.
At this precise moment I can book seats through to and including 14 September 2018. Flights should be bookable for a total of 355 'full' days so you should include the current day in that total rather than adding 355 days to the current date.