SA was invented during the Golden Age, as it signalled the beginning of a new era, and people born in this period were considered to be living ina glorious time. The SA ended when Jafier Heartan took the Golden Throne, thus starting the TA - Third Age.
SA 0 starts after end of the FA which was FA 13000. SA 0 is the 13001 year from the beginning of written times, and lasts up until SA 3400, when the events of the War of Kings begun the Third Age, TA. The end of the Second Age is approx, 15,400 years after the written times.