This handy tool will calculate the build date for a given Android build number. Just input the build number and press Date It! The rules, as per Android developer docs are as follows:
Starting with Cupcake, individual builds are identified with a short build code, e.g. FRF85B.
The first letter is the code name of the release family, e.g. F is Froyo.
The second letter is a branch code that allows Google to identify the exact code branch that the build was made from, and R is by convention the primary release branch.
The next letter and two digits are a date code. The letter counts quarters, with A being Q1 2009. Therefore, F is Q2 2010. The two digits count days within the quarter, so F85 is June 24 2010.
Finally, the last letter identifies individual versions related to the same date code, sequentially starting with A; A is actually implicit and usually omitted for brevity.
The date code is not guaranteed to be the exact date at which a build was made, and it is common that minor variations added to an existing build re-use the same date code as that existing build.