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In the Star Wars universe, time is measured by using "BBY": before battle of Yavin... however, seeings as there was a lot of history before that, including but not limited to:
- The Old Republic
- The New Republic (prequels)
- The First Galactic Empire- & the events of a New Hope incl. battle of Yavin.
How was time measured in that (thousands of years) time period before the battle of Yavin?