Time

Time is the indefinite progress of existence. In Mario's universe, time travel is possible due to the particular structure of time there.

Mechanics

In the Mario universe, there are two dimensions of time: normal-time and meta-time. Time flows forward at a constant speed in both normal-time and meta-time; thus, timelines can be called "time-diagonals". The 2D plane of time can be divided into two halves: the normal-past (the black triangle in the diagrams) and the normal-future. Under normal conditions, it is impossible to travel rightwards into the normal-future, only leftwards into the normal-past. However, once done, a new time-diagonal is established starting at that point in the normal-past and going forever into the normal- and meta-future. The source time-diagonal and this new time-diagonal are "linked", allowing free travel rightwards from the new time-diagonal into the original time-diagonal. There is only one rule for traveling between these two time-diagonals: from the perspective of someone in the time-diagonal you are traveling from, you must have spent a positive amount of time in the other timeline. I.e., it is impossible to travel into the past of your own time-diagonal, no matter what. Time Holes, being an uncontrolled phenomenon, have a second restriction: the time spent away from the origin timeline will equal the time spent in the other timeline.

The structure of time (Adult Mario's perspective)

The structure of time (Baby Mario's perspective)

Simplified diagram of Baby Mario's travel to and from the kart/sports games, starting at the left white square and going along the green lines

One peculiar effect of linked time-diagonals is that certain actions in the past time-diagonal can change things in the future time-diagonal, as seen with E. Gadd and the Hydrogush.