BBY (Before Battle of Yavin) and ABY (After Battle of Yavin) are used to point events on LucasFilm timeline officially. But, do they exist in-universe?
BBY/ABY can't exist prior to Battle of Yavin, but it can exist in ABY to denote past events and even for calendar system. Existence of such calendar system is likely in The New Republic because Battle of Yavin was a golden victory event of rebellions (and, the new republic was their alliance).
Has BBY/ABY ever mentioned in-universe in the canon (probably, in EU)?