The DCU is an unusual, three dimensional, multiverse

We tend to think of time in one dimension: a line which flows inexorably from the past, into the future, in one direction. Time travelers in most universes quickly become used to thinking about time in two dimensions. If you go back in time and make a change, you've created a second timeline which is metalater than the one you originally experienced. In that sense you have a time plane, with a future/past and a meta-future/meta-past. The majority of realities work under this kind of framework. But, there's a lot of diversity in the cosmos. There are always oddball multiverses.

The DC Universe undergoes periodic possibility events which restructure the entire time plane, which it calls a "crisis". So, a time traveler in the DCU may come from a particular time line, and go into the past to change something, creating a separate meta-later timeline 'above' the one they are from. And then one of these "crises" occurs, and both of those timelines are reformed into a completely new metatemporal plane existing "later" in a third dimension, which I hesitate to even name because it's existence is so idiosyncratic to the DC Universe!