I was thinking about the timeline of DC Comics’ Earth-51 (home to the Great Disaster in Countdown to Final Crisis) and trying to wrap my head around what the past and present might mean for a world that’s been created and destroyed twice in as many years, and realized that some of the time paradoxes make much more sense if you consider that there’s more than one kind of time.

Real-world time is, as you’d expect, the time that passes between when two stories are published. For example, it’s been 45 years since Spider-Man first appeared in Amazing Fantasy #15 (Aug. 1962).

In-story time is the time that passes within a story. So even though it’s been 70 years since Superman first appeared on the newsstand, it’s only been 10–15 years since his debut within the DC Universe.

The tension between these two leads to a strange, fluid take on time, which has its own issues.

But then you get into time travel and cosmic retcons, and in-story time can’t quite explain things.

Let’s take Earth-51. At the end of 52, it was revealed that the universe had spontaneously created 51 copies of itself, all identical. This multiverse was then attacked by a creature that essentially ate time, altering these worlds’ histories so that each universe was different from the others.

The universe containing Earth-51 was destroyed in Countdown. A new copy of the main universe then formed to fill the void.

So what was Earth-51 like two years ago? In real-world time, it didn’t exist. In story time—well, that depends. Two years ago in the DC Universe was before Infinite Crisis, which means it was before the new multiverse was created. But each universe has a full history. You could go to Earth-51, go back in time, and it would be there.

Now suppose there’s a third type of time, sitting between real and in-story time. Cosmic time takes place within the story, but tracks retcons, time travelers, etc. Instead of a single timeline, you’d see a series of layers, each layer being the result of some alteration of history.

To switch gears for a moment, let’s look at Back to the Future. Marty McFly travels from 1985 to 1955, changes history so his parents never get married, then changes history so that they do, but under different circumstances. The two possible versions of 1985 are simultaneous, internally to the story. But from the perspective of Marty traveling through time, or of the audience, the version at the end of the film takes place after history has been changed.

For the most part, it’s taking the order of events from real-world time and giving it a meaning inside the fictional world. Now it makes more sense to talk something being the post–“One More Day” version of something that happened before the events of “One More Day.”