Kozuki Toki, mother of Momonosuke, had known Joy Boy from the past. She possessed the Toki Toki no Mi fruit, which allowed her to send people forward in time. Toki had a twin sister who possessed the opposite version of the fruit, one that could send people to the past.
This is how Joy Boy learned about the future: Toki's sister sent him back critical knowledge, allowing him to prepare for the coming battle. He gathered an immense amount of his strongest Haki into the Emeth, knowing that only in Luffy's era would it be possible to defeat Imu.
This is how Joy Boy learned about the future: Toki's sister sent him back critical knowledge, allowing him to prepare for the coming battle. He gathered an immense amount of his strongest Haki into the Emeth, knowing that only in Luffy's era would it be possible to defeat Imu.