Time Travel is the act of traveling from one time period to another. In My Immortal, Eboby travels from the present to the 1980s in order to seduce Tom Satan Bombodil, to prevent him from becoming Voldemort. Traveling through time can be done in several ways.
The first way is to use a combination of a Pensive and a Time Torner. To travel into the past, one must jump into the Pensive. Traveling to the future is then done using a Time Torner. However, if the time traveler forgets to take a Time Torner with them, they are stuck in the past, presumably..
The second way of time travel is done using Morty McFli's blak tim machine, which is appropriately introduced to bring Ebony back to the future, when she forgot her Time Torner. This method seems to be more stable, as it allows the user to travel in both ways. However, the machine itself is a bit fickle, as it seems to disappear after use, but always appears when needed.
When a time traveler dies in a time period other than their own, they will reappear in their native time period. This is evident when Ebony jumped in front of a bullet meant for Lucian. After dying from said bullet, she awakened at the school's norse's office in her own time. Sirius then explained this fact to her.
It is also worth mentioning that in the My Immortal verse, inventions that have not yet been made in the past, will not work there. For example, when Ebony tried to use Amnesia Potion on Tom Satan Bombodil in the 80's, to forcibly make him fall in love with her, the potion did not work, because it wasn't invented yet. It appears this effect only works for magical inventions, as Albert Dumblydore was able to play songs on Ebony's iPod, which wasn't invented in the 80's yet.
One of the most unordthodox conventions of this take on time travel is that people in the past apparently can glimpse into the future (i.e. Lucian knows that his son "won't be friends with Vampire now" after a negative interaction with James, Tom Bombodil knowing that 'amnesia' potion will be invented in the future).
Differences from canon[]
A Prinsive
In the Harry Potter books, time travel is solely done using Time Turners, which can send wizards back in time. It is, however, being established time travel is one of the most dangerous brands of magic. Therefore, the largest amount of time one can travel back is only several hours. (Usually.) For this reason, time travel isn't frequently utilized in the Harry Potter universe and is strictly controlled by the Ministry of Magic.
In My Immortal, time travel is utilized much more freely. It seems the Mystery of Magic has no control over this form of magic (or anything else, for that matter) whatsoever, as multiple characters gain the ability to travel in time quite easily. Time travel is also used more liberal, compared to canon; the dangers of messing with the past aren't mentioned at all. Also, changes to the past are made much more easily. In the original, the only changes one can do to the past already happened aka a stable time loop (again, usually).
The original version of Morty McFli's machine
Also important to note is that the Pensieve, spelled as "Pensive" in My Immortal, isn't a tool used to travel through time at all. Instead, it can make lifelike representations of one's memories, in order to objectively observe these moments. While this can feel like traveling into the past, one cannot interact with these memories. Therefore, it is not really time travel.