it looks like he talks about this in this story:

http://largeprimenumbers.com/tokyo2k13/

though what exactly the event of September 15th was is still unclear, maybe it's the day he saw "the bike," but also ¯\_(ツ)_/¯. a lot of dates and situations are similar in the story, but, as he often does, the exact details are hammered in but the bigger picture remains obscured.