DC continuity is a harsh mistress. Zero Hour came out in 1994, meaning a solid twelve years have gone by in the real world since. Now, take a look at the bottom-left panel, there. Rick and Rex Tyler, Hourman 2 and 1, respectively, are having a chat in a lab existing outside of time created by the third Hourman, a robot from the future. You follow?
Rex was plucked from time just before he was scheduled to die at the hands of Extant at the tail end of Zero Hour, and Rick could pay him visits by way of an hourglass full of theoretical tachyon particles and... wow, does that ever sound ridiculous when it's written out like that.
Anyway, that's not the point. The point is that this comic was published in 2003 and it has a character claiming that the events of Zero Hour took place "a year back in time."
Which means that Gotham City was infected with a deadly virus, leveled by an earthquake, declared federally off-limits and rebuilt in the span of one amazingly crappy year. No Man's Land, by the way, was littered with references to how long it'd lasted, clocking in at hair over a year on its own.
That's not even getting into, say, the founding of the current incarnations of the JLA and JSA and the events of Our Worlds at War, World War 3, Genesis and Day of Judgment.
So, yeah, busy, busy year.