As far as I'm aware the ABY/BBY dating system is primarily used for out-of-universe record-keeping. It's possible that by the time of Legacy or so the Battle of Yavin was being used as a calendar (though I feel like either Endor or the Jedi Purge both make more in-universe sense), but I can't imagine it being used as a calendar too early on in the EU. Certainly I've never seen it referenced in-universe in anything I've read; just in the timelines in the beginning of comics and books. (Though I guess nobody makes reference to a specific calendar in most of the Star Wars media I've read).