First, Crisis on Infinite Earths, then Zero Hour and now Infinite Crisis.
Zero Hour occured about almost ten years after Crisis on the Infinite Earths. Infinite Crisis occured ten years after Zero Hour and twenty years after Crisis on the Infinite Earths.
Gotta say DC has rewritten its continuity more often than any country has rewritten their history.
That all began when the DC comics management has realized the problems with the continuity and needs to reboot it from the scratch.
I have no doubt that the DC comics' continuity will have to face a crisis that propels it through rebooting again in the next ten years.