Actually I've been researching about that. Apparently it happened in the tahun 2047. If anda take the timeline of Finn and Jake which is 3043 and then take away 996 as mentioned in the episode "Simon and Marcie" anda get 2047. It's probably like World War I atau II.The War ended when the cendawan Bomb was dropped. And only few people survived. And the apocalyptic thing, actually I bet the so-called "zombies" might have been caused sejak the bomb, the chemicals, atau maybe a disease atau something. I'm not sure which happened first though :) Wait, actually I think it's because of the chemicals and wastes mostly. Because when Shoko from the episode "The Vault" she fell in the river and was mutated.