Oh well, Star Trek *is* fiction, right?
Certainly. Like, I doubt that anybody was really upset that Captain Christopher wasn't a real person or that Roswell wasn't really Ferengi.
But with fiction it's great if it's at least relatable, meaning that any kind of mass holocaust or crazy happening ultimately has to be in "the future." The dates aren't really that important, and can probably be fluid.
It's almost like how every Terminator sequel had to constantly shift the date back. In the original, the war was supposed to come "soon", but I guess that really meant 12 years. Then when they actually gave that date, it was pushed back to 2003 in T3. Then the show pushed it even further to 2011.
To keep it relatable to modern audiences, it simply has to be in the future. They're not going to say, "Hey, remember that huge devastating war in the 90's?" And with the way JJ's creative team operates, I wouldn't entirely rule out some sort of flashback to the end of the war and Khan's escape. That could be really interesting.