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?
What does a.t.b. stand for?
It stands for "Ascension Throne Britannia", named after the "date the first Celtic super-king, Eowyn, was elected by the united British tribes to battle Rome, becoming the first member of the modern Britannian imperial family."{1} 55 B.C. is equivalent to 1 a.t.b.
How can we convert between a.t.b. and R.Y.?
The Revolution Year system (used by the EU in Code Geass) is based on the French Revolution (start date) of 1789 A.D.{2}{3} Since 55 B.C. is 1 a.t.b., we can figure out the actual conversion formula:
0 R.Y. = 1789 A.D. = 1845 a.t.b.
If you want a simple conversion between a.t.b. and R.Y., just some simple math:
R.Y. = a.t.b. - 1845
Which Revolution Year would 2010 a.t.b. be?
Based on the conversion above, a.t.b. 2010 is equivalent to R.Y 165.
AD = atb - 56, but if 55 BC = 1 atb, then 1 BC = 55 atb, and 1 AD = 56 ATB (there's no such year as "0 AD"), hence AD = atb - 55.
At the beginning of the second episode of Akito the Exiled, the following is said:
Revolution Year 228. It has been seven years since the Empire of Britannia invaded Japan and renamed it Area 11.
Because the invasion of Japan happened in 2010 a.t.b, it can be deduced that the offset, at least in the time frame of the series, between a.t.b and R.T is:
2010 + 7 - 228 = 1789
So it appears that R.Y. 0 would correspond to 1789 a.t.b.
