On time and various related subjects within Homestuck

The nature of time manipulation and timelines within Homestuck seems, on its surface, to be relatively simple, but when one looks deeper, they will find it is anything but.
The most major aspect of timelines is known as the "alpha" timeline. This timeline is the one where everything happens precisely as the Incipisphere had planned, and it alone claims both the birth and death of Lord English's servant, Doc Scratch. While those within the timeline feel as though they have total free will, deviation from the scripted chronology is for the most part impossible; any attempts to do so are simply part of its script. Any being who can honestly claim omniscience will have total knowledge of this timeline.
However, there are circumstances in which it is possible to break the script. If one gleans surefire knowledge of the future - that is to say, a guess is insufficient, but knowledge gained from precognitive abilities, time travel and the like would be enough (knowledge gained via dreamself is generally not enough to allow this, since it is never fully remembered beyond vague, dreamlike memories) - they can consciously deviate from the alpha timeline, which causes their timeline to splinter off into an offshoot timeline. In addition, those granted this knowledge can unconsciously make tiny deviations from it, which alone are not enough to qualify for an offshoot (since the alpha timeline is defined not by the exact nature of its contents but by major events) but which, though a sort of butterfly effect, can cause events to go differently than they otherwise should, prompting an offshoot. A prime example of this is Aradia, and her precognitive abilities: because of her knowledge of the future, she caused many, many offshoot timelines, though these would later prove critical.
This last point leads into another, which is that these offshoot timelines are still tied to the alpha, at least in part. While for the most part the events occurring within them are outside of its control, it does claim dominion over their creation - paradoxically, the deviation from the script is in fact scripted. In addition, should a person return from an offshoot to the alpha, they will return to its control, as Dave did when returning from his alternate future.
However, such travel is not to be taken lightly. Those travellers who return from an offshoot timeline to the alpha are marked for an early death. This is what has led to the common name of "doomed timelines" being used to refer to offshoot timelines. There are workarounds, in that if such a traveller is to undergo any process that would grant immortality - such as merging with a Kernelsprite - they will, of course, gain this immortality, and though they will inevitably meet with at least one event that would normally be sufficient to kill them, they will live on, and thereafter will no longer qualify as "doomed".
Those who inhabit the alpha timeline may freely communicate with one another, of course. However, once a timeline splinters off, one becomes unable to contact anyone who does not currently reside in the same timeline as oneself. Upon returning to the alpha, or indeed any other timeline, one can then communicate with the inhabitants of that timeline - at the expense of contact with their native timeline - but will, of course, then gain the "doomed" status.
Time manipulation is highly inadvisable to anyone unaccompanied by a high-level Time player, since one can very quickly splinter off one's own timeline if not careful. Even those with a Time player should be highly careful, unless said player has ascended to the God Tier and gained the accompanying intrinsic knowledge of timelines. Those who do attempt it will have to procure, by some means, an artefact of time, such as the Timetables or the Music Box Time Machine - another reason it is preferable to be aided by a Time player, who will come across these items on the course of their session.
Once a time artefact has been obtained, one can travel into either the future or the past. If one should travel into the past, or to the future and then return to the present or past, they can trigger one of two events: a stable or unstable time loop.
Loop stability is maintained when one lives up to every instance of their future self, which is to say that they eventually become that instance, performing every action that they did/will do. This takes, in the words of the Knight of Time, "a mixture of careful planning and rolling with your gut", "trusting [that] the decisions you make are the ones you were always meant to". If the Time player has ascended to the God Tier (granting them full knowledge of the timeline) or has precognitive abilities, they can make use of these to aid them, but otherwise there is no way to ensure total stability. If a loop becomes unstable, this causes an interesting instance of a doomed timeline in which the only inhabitant always remains in the alpha - they are likely to die very shortly after, however. This is why it is inadvisable to attempt time travel without full knowledge of your timeline, assuming one is not in fact fond of dying.
As alluded to earlier, an omniscient being will know the alpha timeline in its entirety, with the exception of events that require their intervention (this is to ensure that they act to cause outcomes they have seen, thus continuing the alpha timeline on its script). While some omniscient beings may have knowledge of timelines that are not the alpha, this is not a necessary factor in omniscience, nor do those who have mastery of offshoot timelines but not the alpha qualify as omniscient (for instance, a fully-realized Seer of Mind can, through use of their powers, gain knowledge of any offshoot timeline, they do not necessarily know the alpha timeline, and thus do not qualify as omniscient - in addition, they will likely have no knowledge of which timeline is the alpha and which is a hypothetical offshoot, and thus cannot act accordingly.)
The planet in the Incipisphere reserved for the Hero of Time will always have at least one attribute that their species associates with time in some manner (the Knight of Time had clockwork, as in a clock, which measures time; the Maid had quartz, which is used to allow a variety of clock to function.) The time artefact for this player will always be related in some way to this attribute (the Knight's had clockwork components, while the Maid's was made entirely of quartz) and the Beat Mesa, the structure which enables the Scratch, will be similar, though it will have significantly more temporal energy bound to its structure, to allow for the much greater and larger-scale time manipulation involved.
Distinct universes will inhabit seperate temporal bubbles, and thus their chronology is incompatible - there can be no "before" or "after" where two seperate timelines are involved, and offshoot timelines are only relevant in their universe of origin. Therefore, if one can find a way, they can look at any point they like on another universe's timeline, without the aid of any sort of time travel. Communicating between universes often leads to troubles in this way, as one can easily influence its outcomes unconsciously if they have looked into the "future" relative to the moment of contact.
This has the interesting side-effect that, if a group of players is successful and creates a new universe, and the players do not then enter the universe through the endgame door (doing so places them on the timeline at a point determined by whatever cosmic forces dictate the alpha timeline), the created universe can interact with its creator, even mere moments after its creation, at any point in its own timeline.
There also exists the space between universes, which is commonly known as the "Furthest Ring". This is a place where no reliable time flows, nor does space reliably curve (though this will not be explored in detail, since we are currently focusing on time, not space.) Unless one has mastered the flow of time or space in its entirety - which is to say, unless one is, or is accompanied by, a God Tier Hero of Time or Space - it is nearly impossible to navigate this "space". The further into it you move, the less reliably time flows, and if one is not careful, they may find themselves many eons in the past or future relative to the time they entered, with similar principles applying to space.
An interesting sub-phenomenon in the spacetime of the Furthest Ring is that of the "dream bubbles", small pockets of more reliable spacetime created by the Ring's inhabitants. While they technically have more reliable timelines, not changing randomly, it is still highly flexible and changes based on its inhabitants' subconscious(es). This simply means that the "current" time in a bubble is based on the time at which any given memory occurred. This is difficult to control precisely, and so it is better simply to allow the bubble to shift as it does naturally, unless, again, accompanied by a god of Space or of Time.
Since both the Ring and dream bubbles are outside of any reliable chronology, this also excludes them from the alpha timeline's control. However, this does not make it an offshoot timeline - rather, it is entirely independent of the timeline system, the only known example of any such thing. This means that even omniscient beings do not know entirely what will happen, or has happened, or is happening within the Ring, nor indeed can any person know, even a Seer of Mind or any other person who normally has extensive knowledge. However, there are areas of the Ring that will naturally be inside the bounds of the knowledge of certain beings: any First Guardian, for instance, will know the entire past, present and future of the Green Sun, which exists within the Ring but grants them their power and omniscience. Any Derse dreamers will know a limited amount about the very border of the Ring, including being able to hear the Horrorterrors' (the Ring's inhabitants) speech. These players can gain knowledge of the future should the Horrorterrors choose to give it to them, bringing us neatly back to where we began.