For a while glitches such as ghast resetting and javaw resetting have been used in glitched categories, which allow you to revert the world to the previous save point, with little to no increase in the igt. These can be used to repeat elements of rng or to scout a seed in rs categories. You can watch any% ss no time travel 1.9-1.13 runs to see examples of ghast resetting, and any% rs no time travel 1.14+ runs to see examples of javaw resetting. These glitches on their own are NOT time travel.

Time travel occurs when the igt is reverted to a time A, but some change to the world made after time A is saved. The main time travel glitch is done (roughly) by:

1. Modifying a chunk
2. Unloading the modified chunk (this saves the chunk)
3. Stopping javaw.exe through task manger (this reverts all unsaved chunks and the igt to their most recent save point).