2009:
The near apocalypse of '09' was the penultimate battle between Batman & Ra's Al Ghul. This event has never been seen, as no episodes of the DCAU actually take place in 2009. But it's been referenced twice by the 2nd Batman, Terry McGinnis:
Batman Beyond episode "Out of the Past": "I've heard about you too. You're the daughter of Ra's Al Ghul; one of the old man's worst enemies from way back. You helped the original Batman fight Ra's one last time in the near apocalypse of '09'. For the last couple of decades you've been putting your father's money to work cleaning up the mess you made." - Terry McGinnis to Talia.
JLU episode "Epilogue": "My dad wasn't a superhero. He didn't save the world from the near apocalypse of 09." - Terry McGinnis in an imagined conversation with Bruce Wayne.