Star Wars date question

Hoping someone can help me out here.

Years in Star Wars are relative to the Battle of Yavin -- BBY (before
battle of Yavin) and ABY (after...) -- much like our B.C. and A.D. 

Does anybody know how, in the fictional Star Wars universe, dates were
measured BEFORE the battle occurred?  It's not like they knew the battle
was coming decades in advance, so they could say, e.g., it's currently
the year 124 BBY.  How did they count years before the battle? 

thanks
DB

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