BA US counts your first twelve months as a 'year' and then switches to a calendar year. This is a relatively fair way of assigning status. If you move your account from elsewhere to the US they apply a similar rule - I was granted about eighteen months of 'free' status because I had a mid-year move. It does take a few months for them to fully transfer your account, so redemption has to be put on hold.

As far as I'm aware, BA global [may vary by region] runs on a twelve-month cycle determined by your joining/tier qualification date.