- “TG: the thing with time travel is
TG: you cant overthink it
TG: just roll with it and see what happens
TG: and above all try not to do anything retarded”
There are multitude of universes and timelines in Homestuck; pre- and post-scratch Earth, pre- and post-scratch Alternia, the pre- and post-scratch kids' Incipisphere, the pre- and post-scratch trolls' Incipisphere, and cherubs' universe and Incipisphere. Each of these universes has an alpha timeline, which may interact with the other universes. These ten timelines are non-parallel; with the proper equipment or software, you can travel, send items, or communicate with other timelines at any point in its history.
The alpha timelines must be internally consistent, with all time travel being done in the form of stable time loops. When this rule is violated, a doomed timeline is created, splitting off from the main timeline. As the link between normal universes and the incipispheres they spawn are closely intertwined, doomed timelines often span both of them, such as in Davesprite's timeline, or one alternate future Aradia's timeline. This could imply that a doomed timeline could span any number of universes, or just that doomed timelines in any incipisphere affect its parent universe.
Doomed timelines cannot interact with universes outside of them. The doomed player may contribute to the alpha timeline in a constructive way for short while, but will soon die. Dave speculates that loopholes, such as prototyping or ascending to God tier in lieu of the alpha player, may extend the doomed player's life a little longer, but will not spare them from their ultimate fate. The timeline continues to exist, dooming anyone who remains in it. It is implied that these timelines, like the players that come from them, are destined to somehow be destroyed.
Another important part is the Furthest Ring. It is a void separating the sessions, inhabited by Horrorterrors and sleeping/dead players after losing dream selves. Time and space do not work there in a linear or comprehensive way, causing problems for both players and readers alike.
In the Homestuck Midnight Crew intermission, time shenanigans appear in abundance. The Felt, a time-manipulation based gang, are being attacked by the Midnight Crew. The Felt are equipped with many items with influence over time, some of which the Midnight Crew manage to get their hands on and abuse in a thoroughly confusing and migraine-inducing manner.
Trace follows people's Past Trails and can interact with them. How this works is really confusing.
Fin follows people's Future Trails and, like Trace, can interact with them. He ends up following Clubs Deuce's future trail, while CD follows the trail of blood left by Fin as he follows CD's future trail as he follows the trail of blood left by Fin. Also confusing.
The trolls are well known for their time shenanigans. They have the ability to access any part of the kids' timeline before the scratch, allowing for random communication and troublesome meddling.
They troll mostly linearly, but most of them are not above jumping around random points on the timeline. Karkat began trolling backwards but eventually returned to troll linearly, while Terezi makes a point out of refusing to troll non-linearly.
The trolls themselves are not immune to these shenanigans; Karkat attempts to set up a pan-temporal bulletin board for the use of his team, but this meets about the same level of success as his attempts to troll John.
Kanaya pulls Rose's GameFaqs guide off of a server in the Furthest Ring. This helps the trolls in completing their session, which creates the kids' universe, after which they start playing Sburb, and Rose writes the GameFaqs guide and sends it to the server, where Kanaya takes it.
While originally the Exiles were simply depicted in a time way ahead of the Homestuck Kids, they later confused matters thanks to past versions of themselves, who were later exiled and hence become involved in messing with the past with the kids, who in turn mess with their present selves, who later became the exiles and started messing with the past with the kids, who in turn messed with... you get the point. They have also been responsible for sending Package through time and space.
An alternate timeline existed in the main Homestuck story due to Terezi meddling with the flow of things. While travelling around the Land of Wind and Shade, Terezi directs John back to his house, where she gets him to fix his Rocket Pack so he can simply fly to the seventh gate and fight Typheus while he's asleep, as opposed to playing the game normally. From what has been told through Dave and Rose's pesterlogs, John was killed by Typheus as he was too low on his Echeladder, and as such, he couldn't connect to Jade and save her island from the meteor, so she was presumed dead. Future Dave went back in time using his Timetables to prevent John from going through with the suicide mission and prototype his Sprite with himself instead of Lil Cal.
By doing this, the now "Davesprite" caused this alternate timeline to cease existing, but not before Rose somehow fused her future dream self with the current timeline, as the dream versions of the kids appear to exist outside of the normal space-time continuum.
When the Draconian Dignitary went to the Land of Heat and Clockwork to steal Dave's copy of Rose's books, Dave attempted to stop the theft by using his Timetables to travel back to the moment of the theft. The Dignitary effortlessly murdered Dave, leaving Dave in the present to discover his own corpse and decide against time traveling to stop the theft, averting another bad future.
Dave's Sburb Betas were stumbled upon by Jade, being spat out of a time capsule inside the Frog Temple. It was later revealed that these betas were also taken by the Draconian Dignitary, and carelessly discarded. They landed inside the time capsule seed, which then set to open in 413 million years, and was then sent off in a meteor to about 413 million years back in time, to crash at Jade's location.
Dave would later discover the LOHAC Stock Exchange and begin to repeatedly reverse time to abuse the system by using his knowledge of the future to win and greatly multiply his stock of Boondollars. According to him, this involves a lot of stable time loops and multiple Daves running around at the same time (because otherwise that means alternate timelines and dead Daves).
When John puts his hand into a treasure supposed to be able to beat Lord English, he gained the ability to travel through time and affect the alpha timeline without creating doomed timelines. John proceeded to make multiple changes to the alpha timeline from the way it originally proceeded, to the point where he ends up undoing a multitude of deaths with a little help. There are some situations in which his retcons create paradoxes, such as his interaction with another of his retconning selves to stop them from interfering, but the ability has the ex-machina-like ability to fix paradoxes and such without creating a doomed timeline.
As Sburb had only come into beta at the time of the story's beginning, this is set to explode into a self-causing wellspring of confusion.
It seems that the Guardians and the Homestuck Kids were created by present day John in the ectobiology lab from paradox versions of the Guardians past selves, turned into clones, then mixed up resulting in a situation that is every kind of weird shit you could imagine.
Other examples in Homestuck
The Bunny, first encountered as a gift by Dave to John, was sent back in time with baby Rose, who fixed it up and was going to send it to John as a gift. John retrieved the package and sent the modified bunny back with baby Jade, who improved it with help from her penpal and sent it to John as a gift. Before John received it a third time, Jack Noir got his hands on it, causing even more time shenanigans.
It's a safe bet that Bro's rocket board was alchemized with John's Rocket Pack and something else before ending up in Bro's hands. The Crosbytop traveled from the kids' Medium to Alternia when Jack Noir entered the trolls' session with Lil Cal (containing Dad's wallet) and Past Aradia discovered it. Vriska's rocket boots may have taken the same route.
In order to save the post-scratch kids' session, Dirk is forced to sendificate a bucket into the past, followed by sendificating his own head to Jake, killing himself in the process. Jake, woken by having water thrown on him, then kisses Dirk's head, waking Dirk's dream self, who then (with dream Roxy) enters the lotus time capsule. On emerging from the capsule and picking Jane up, Dirk catches the bucket he sent, fills it with water from the sea, and then throws the water on Jake to wake him up so that he can kiss Dirk's head.
An Orderly Timeline
The MSPA Wiki has made an attempt to make these time shenanigans more understandable by ordering all the major events of Homestuck chronologically.
Pickle Inspector's Many Clones
Many clones of Pickle Inspector were created over the course of Problem Sleuth, including Past PI, Future PI, Past-Future PI and Future-Future PI. This leads to a LOT of time shenanigans, which is detailed in their respective articles. Pickle Inspector is also the progenitor of the Hippo Inspectors, which, on a mission from Godhead PI, go back in time and clone innumerably, becoming matter and creating the entirety of existence. In other words, because of time shenanigans Pickle Inspector becomes quite indirectly responsible for all time shenanigans ever.