In the first two seasons of Code Geass, dates are mentioned with "a.t.b.". I took this screenshot from first episode of season one:
However, in Code Geass: Akito the Exiled, dates are mentioned as R.Y. (Revolution Year). This is a screenshot from the Akito the Exiled trailer:
So, first question: What does a.t.b. stand for?
And, also: How can we convert between a.t.b. and R.Y.? Which Revolution Year would 2010 a.t.b. be?