It's not quite that simple, but it's based on a parsec -
http://en.wikipedia.org/wiki/Parsec
which is an easy calculation to make.
Also, this - "Measurements of the cosmic background radiation give the cooling time of the universe since the Big Bang, and measurements of the expansion rate of the universe can be used to calculate its approximate age by extrapolating backwards in time."
Since the universe expands at a more or less constant rate the age of the universe is not too hard to extrapolate.