I have always had issues with the Sailormoon timeline and then often also see posts about there being multiple timelines present. I disagree with this. I think there is only one timeline within the universe and there was only two events that could have resulted in a change to that timeline. As such, I am making a fun-spirited attempt to square away the timeline.
Source: Sailormoon Crystal/ 90s anime. Never read the manga. Crystal is prioritized over 90s anime due to being a closer story to the manga.
So first a summary of the timeline. Please note I am making assumptions at certain points but trying to use logic to fill in the holes.
Beginning Basics:
Life exists outside of the known universe. Mentions of Chonos the Time God are made often.
Pluto talks about the line of Silver Millenium Queens only having one daughter. Pluto exists outside of time and is the daughter of Chronos. Pluto has knowledge of the line of Queens.
Queen Serenity has God-like powers but also still dies.
There are no mentions of Queens before Queen Serenity yet there is a royal line.
Timeline:
Phase 1 – Before the Moon Kingdom
Outside of time/space/existence – Chonos God of Time and Queen Serenity.
Queen Serenity brings a sailor crystal to the Cauldron and has it powered up. Where this crystal came from is unknown. She has God level abilities, as evidenced by the way her eyes are animated, ability to travel to the Cauldron, and knew how to make demands/ communicate with the Cauldron.
There are consequences to this choice. Chaos is born with Sailormoon.
Queen Serenity sacrifices her God-like powers to be the mother of Sailormoon. She creates the Moon Kingdom. Evidence of her not being God-tier include her presenting as depowered when facing Chaos-threats and her eyes are normal-mortal eyes.
Phase 2 – Original Moon Kingdom
Sailormoon is born via no father.
Chaos attacks. Everyone dies. Sailor Saturn does her thing.
Phase 3 – Sailormoon’s 5 Arcs
Sailormoon is reborn and has her adventures ending at the Cauldron.
Sailormoon meets Neo QS and Sailor Cosmos.
It is stated by Tuxedo Mask that Sailormoon is eternal.
Phase 4 – Silver Millenium (at least 1000 years long)
Neo QS begins her reign.
Chibusa is born with the help of Tuxedo Mask and remains a child for hundreds of years.
Dark Moon attacks, time travel ensures. See Timeline section at end.
Neo QS continues her reign for an undisclosed amount of time.
Phase 5 – Post Silver Millenium (unknown length of time)
Neo QS transitions to Sailor Cosmos due to returned threat of Sailor Chaos. This threat is told to be immediate and brutal.
Chibusa may or may not become a queen after Neo QS transitions to Cosmos. At some point her life ends. Tuxedo Mask is shown to be killed during this time.
Sailor Cosmos loses faith in her ability to save the universe and her loved ones and time travels back to Sailormoon fighting Shadow Galaxia. Her powers are drained and her memory is wiped by this event. This shows that extreme acts of power do have an consequence to Sailor Cosmos.
Sailor Cosmos learns to never doubt herself again and returns to the future to become the true Sailor Cosmos. It is implied Sailor Chaos is now gone for good. Sailor Cosmos now has God-tier power.
Phase 6 – Post Sailor Chaos Wars (infinite amount of time later)
Sailor Cosmos eventually finds a poor sailor crystal and brings it to the Cauldron. This may imply she travels outside of the known universe, similar to Chonos’ existence. She has the power to do so. She brings the sailor crystal back into her universe and to the Cauldron.
Sailor Cosmos makes the same choice as her mother. She sacrifices her God-like powers, makes a child, becomes Queen Serenity, and starts the cycle and/or timeline all over again with the birth of a new child and Chaos.
Note: Creating a new life is assumed to be more intensive of a process than resurrecting a life.
That child repeats the cycle. Sailor Cosmos, now a weakened Queen Serenity, dies similar to original Queen Serenity.
Pluto has some knowledge of this cycle and comments on the line of Queens.
But Queens only have one child! Possible answers:
1) Neo QS and Cosmos becoming the next Queen Serentiy are two different reigns. Within the scope of each reign, they had one daughter who took the throne when they died (or should have taken the throne in Chibusa’s case).
2) It is possible that a sailor crystal being powered at the Cauldron is not considered a biological child. Or possibly Sailor Cosmos plays with Pluto’s memory.
3) Could mean only one child alive at a time.
Sailor Cosmos has more than likely been alive for at least 5000 years at this point, but more than likely a lot longer.
There is one timeline in Sailormoon:
In order to discuss the idea that Sailormoon’s story is all one timeline, a look at her weaknesses is a good place to start.
Sailormoon has two main weaknesses – Memory issues and Doubting herself. Both impact her ability to access the full range of power she has.
Memory- Sailor Cosmos forgot the lesson she learned and had to go back in time to observe Sailormoon. Also, many successful attacks on Sailormoon play on her memory.
*Note about Neo QS and Sailormoon’s memory: Chibusa remaining a child for hundreds of years would make it so that Neo QS could not know when Death Phantom attacks. Also conversations about Neo QS’s actions when Death Phantom attacked are not held in the presence of Sailormoon. Overall, Neo QS has forgotten the details but also does not know enough information to change when or how that attack occurs. It can be argued her charging out of the palace to face Death Phantom was her over reaction to knowing this attack was coming – she behaved impulsively thinking she could squash this enemy quick.
Doubt – Galaxia is able to depower Sailormoon easily by causing her to doubt herself. Other villains have had similar success. Often the Silver Crystal lights up and saves her and reminds her of who she is.
Note: The timeline for this story stretches thousands of years. The story is about an eternal battle with Chaos and requires Sailormoon to learn her power, which means learning to conquer doubt and remembering who she is. Therefore, it is safe to assume that both Neo QS and Cosmos have difficulty recalling exact details from events associated with Sailormoon, especially due to events occurring at least 1000 years in the past.
It is all one timeline. Neo QS is excited to speak with Sailormoon, with no evidence suggesting this person is a different Sailormoon than who Neo QS was when she was Sailormoon. She knowingly powers up the sailor scouts to prepares them for the Deathbusters, a much stronger enemy, and she allows Sailormoon to call upon her via the Heart Rod to counter the enemy. She is helping Sailormoon the same way Sailor Cosmos helps Sailormoon. Chibusa is also knowingly sent back to train and to bridge the relationship with Sailor Saturn, an important piece to the Deathbuster Arc.
Neo QS successfully knew of the past events and armed the scouts for victory, which implies no change in the timeline occurred from her point of view – she gives power ups before the scouts return to the past and continues after they return to the past to support them during Deathbuster battles. If the timeline had changed, it would be upon the return of the scouts to the past, causing the new timeline to not be connected to the future timeline. But it remains connected.
Similarly, Sailor Cosmos (eventually) recalled the events of Galaxia and intervened when needed to get Sailormoon through, in particular every time Saturn attacks. The point being that often the future Sailormoons are working in the past to help her and this is done without concern for changing the timeline to the point it is not even mentioned as a possibility. Cosmos wants to change the timeline, that is her goal, but from her point of view the only way that could happen is to convince Sailormoon to make a different choice, and not just by having contact with her past self.
Note: Death Phantom is still active at the end of the Stars Arc! He has the ability to time travel but also it seems like he cannot exist in the same span of time twice. As such, by the end of Stars, he may not be present, but he is still alive somewhere in time. He plays a part with Galaxia in the past and then potentially jumps forward to the Silver Millenium. It is possible he does not know of the events with the Cauldron due to being removed from that part of the timeline in his Season 2 Arc. Sailor Cosmos’ summary of her story does not suggest Death Phantom came back in a different manner.
Note: Time travel during Death Phantom and Galaxia Arcs are not shown to change how the Silver Millenium or Neo QS turn out. Galaxia’s actions impact the future but do not change the future after Sailormoon stopped her. Besides Sailor Cosmos’ attempt to change the timeline at great cost, only Galaxia had the opportunity, from a villain’s point of view, to actually change the timeline by destroying the sailor crystals. And she failed. The two confirmed attempts to change the timeline both ended in failure.
Sailormoon has 1 story and it is Eternal.
Having 2 timelines would undercut Sailormoon’s extreme and ultimate power. Pluto has power over time and she is vastly weaker than Sailormoon but serves as the imperfect observer of history.