All of the dates I’ve seen are based on the Battle of Yavin calendar. Fans came up with it to describe how many years before and after the destruction of the Death Star happens had passed. Episode III, then, is (the same age of Luke) before the Battle.
As “years” go, there’s some 20000 years of written history in the EU, I think. TGIF, though. … :dubious:
I don’t think they’ve ever given us any kind of Standard Calendar. But like The Hook said, the Republic lasted for tens of thousands of years. Also, in addition to the Standard Calendar, you would probably have a local calendar on each inhabited planet.
Incidentally, Revenge of the Sith is set 20 years before A New Hope. Attack of the Clones is set 3 years before Sith, and The Phantom Menace was 10 years before Clones.