Year System:
BGF - Before Garden Fall (aka when the Heartless Invasion started)
AGF - After Garden Fall


Timeline

??? BGF (longass time ago) - First Keyblade War occurs. Destroys the original world. From the hearts of children, thousands of smaller worlds are created. Unknown if any children were harmed in this process!

~60-80 BFG - Xehanort is born on Destiny Islands. At some unknown point of time, he is discovered to have potential to be a keyblade wielder and leaves the islands for good to train alongside Eraqus with an unnamed Master.

~40-60 BFG - Eraqus and Xehanort pass the Mark of Mastery. Eraqus becomes the heir to Land of Departure, while Xehanort travels the worlds and becomes a seeker.

~17 BGF - Ventus is born (location is unknown). At some unknown point of time, he is discovered to have the potential to be a keyblade wielder and is taken on as Master Xehanort's apprentice.

~17 BGF - Lea and Isa are born in Radiant Garden.

7 BGF - Riku is born on Destiny Islands.

~6 BGF - Master Xehanort, now a historical re-enactor gone overboard, decides to see if he can make a second Keyblade War to see the conclusion of it. He starts to push his apprentice, Ventus, into darkness to see if he can create the χ-blade out of him.

6 BGF - Xehanort, impatient with Ventus's lack of progress, cuts Ventus's heart in half. Vanitas is born in the Keyblade Graveyard. Ventus starts to die, and Xehanort brings him to the Destiny Islands so he can live out his last days in peace

6 BGF - Sora is born on Destiny Islands.