Why’s it called 358/2?

Mmm SPOILERS:

It’s basically the days that Roxas is active after Sora turned into a heartless to when the beginning of KHII starts. (KHII starts with Roxas being himself for a week. Take that and minus from a normal year and you get 358 days.)

Now the real spoilerish part is the additional Org XIII member. The game is seen through both that character and Roxas. That’s where the 2 comes from.