As all of you know the Sailor Moon timeline spans around two years: from Usagi and the inner senshis' eighth grade till tenth grade. While in the manga and probably the intention of the anime, each arc span half a year, or at least two arcs make up a year. However, there are clear evidences that each season/arc of the anime span a year.  Every season(if I remember correctly for SuperS as well), their school uniforms change from long sleeves for winter/early spring to short sleeves for summer, then back to long sleeves for late autumn/winter. The changes in their uniforms incidate a complete school year(the Japanese start school in April), but they don't get into high school/tenth grade until Stars. On the other hand, the change of seasons is shown, where summer occur in the middle of the season and winter towards the end.
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Here's my theory: it's possible that time is reserved back to the start of eighth grade after the final battle of the first season, so both classic and R span the whole year of eighth grade due to time reversal. They move onto nineth grade in S and SuperS. I'm less confident of the S/SuperS timeline theory, I might have to rewatch some bits of SuperS. Either time is reversed again after the final battle in S, or everything in SuperS is packed between the end of S and before the start of tenth grade. Both possibilities suggested are rather unlikely. The timeline in Stars is fine on its own.
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As for the movies, the R movie probably happens after Chibiusa discovered their identities and before the group set off for Crystal Tokyo. The S movie either set before the Hotaru arc and after the Holy Grail appeared or around Viluy's episode as there is snowfall, I favour the latter. Someone might argue that Hotaru isn't feature or even mentioned in the movie. After all, their lives don't surround Hotaru's, therefore it's not particularly surprising that she's completely omitted. I'm pretty sure the SuperS movie is set after the end of SuperS and before Stars, as Pegasus isn't summoned, implying that he's no longer live in Chibiusa's dreams.
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***Fairly off topic zone***
Speaking of the movies, I can't get some of the anomalies off my mind.  Chibiusa and the Outer Senshis' sudden loyalty and obedience to Sailor Moon during R and S movies respectively is rather suspicious. Why did the Outers come back to fight all of a sudden in SuperS movie?(perhaps the dream black hole is from another galaxy?) How did they get to the marzipan castle without hopping onto the airship?(or they become a stowaway? Haha) And more importantly, why is Pluto here? She's obviously dead, or rather, her civilian form died back in S. She no longer has a physical form in the normal human world and returned to her position as a time guardian.
///Fairly off topic zone///
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I guess the timeline would be more logical in the manga as the story is a lot more fast-paced, the anime tried to follow the general manga timeline but actually lengthened it due to the filler episodes. Except "Kaguya-hime no Koibito"(the story which the S movie was based on) can't really fit into the manga timeline of the Infinity arc, as the arc probably occured in during the first half of nineth grade. More logically it occurs during the Dream arc, due to the outer senshis have already moved out of Juuban and the story is set around winter.
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Like any forums, all opinions are welcomed! If anyone wants to suggest other possible explainations about the oddities suggested above, I'd be more than happy to read it and brainstorm for more ideas, and so do everyone.

Mod Note:
Hey sweetie! I'm gonna move this over to the Theories and Debates section since what your discussing would fit there better

~Mystery <3

I like to think the SuperS movie starts right after the events of the first episode of StarS where they meet the outers again. But then again how is saturn not there... oh man. Tomorrow I'm gonna make a long post about this.

I'm stick to after SuperS and before Stars, as the outers haven't get their upgrades and Mamoru isn't bewitched. Now, I disagree with my theory about Kaguya-hime in the manga timeline, as the inners are still using planet power makeup. :/

I agree with you : the manga is more fast-pieced than the anime , because of its filler episodes but that doesn't explain why Hotaru don't makes any appearance  in the S Movie . Maybe the Movie happens right after episode 112 ?

The time resetting before the start of R was pretty much confirmed by one of the show runners (Ikuhara) as being their thinking of what happened when they were making the show.  They just made a mess of it (I'll get to that later).

Also yes S and Super S are supposed to take place in the same year, only no that does not remotely work without another reset taking place around the end of S otherwise there are two of every season in that year and ages/grades do not work with Stars.   The best way I can figure it would be for the second temporal reset to have happened during the S movie.

I'm going with Genvid's assumption that the S Movie takes place after episode 122 since it has to be before Pluto dies and also takes place during winter.  So in the S Movie Snow Kaguya shows up and does major damage to the whole planet.  I mean really you freeze the whole planet like that and a lot of people/plants/animals are going to die
Sailor Moon uses the full power of the Silver Crystal to fix it with a massive wave of energy washing over the whole planet.  She obviously doesn't just thaw things as that would just mean everything is flooded and still left a ton of damage and dead everything.   So maybe the way she fixes it is with a second temporal reset of a year?  No there is no logical reason why she would turn back the clock a full year instead of just like a day, but then again Sailor Moon was probably not trying to mess with time only to undo the damage...so with her power greatly amplified up by all the other Senshi and her thoughts not being specific maybe it just kind of happened.
Only I'm thinking the Deathbusters since they aren't from this universe, maybe they were somehow immune to the Silver Crystals temporal effect.  Which means everything they did still had to have happened, and so the Senshi's memories of those events still exist too although there could still be changes to certain things on their side.  It's just non-deathbuster related things like grades and ages and all that which went back a year?

Super S Movie is messy with Pluto appearing...we don't actually see her first re-encounter with Uranus/Neptune so they could have been shocked at her appearance?  Meanwhile I'm not sure they ever bothered to tell the Inners she died as they had no reaction of surprise when she showed up in Stars either.  We also don't see when Pluto leaves...so I mean I guess she could have done something like a mindwipe, or else told Uranus/Neptune that this was a one time thing and she would not be back leading to their shock at her appearance in Stars?
As far as why she returns, if we assume that it was Chibiusa's (who was not supposed to be present in the original timeline) energy that was going to give Queen Badianne what she needed to devour the world then it makes sense that it could have been a one time temporal crisis that the guardian of time would need to fix?

And even with all that there is still an mess because even if you assume TWO temporal resets of a year took place...Usagi STILL should have been celebrating her 16th birthday (June 30th) during S.  The only way to get that to work out is to assume that Usagi actually rounds up her age by almost 4 months at the start of the series and is actually 13 for those first twelve episodes, with her then celebrating her 14th birthday after that point  (and celebrating it again somewhere before episode 20 of R post time reset).

So with all that said timeline wise that would leave something roughly like this:

Sailor Moon
April 1992 (Usagi 13 - Grade 8- )
Winter 1992  Usagi 14 - Grade 8- )

TIME RESET

Sailor Moon R
April 1992  (Usagi 13 - Grade 8- )
January 1993 (Usagi 14 - Grade 8- )

R Movie
January 1993
(Up for debate whether this happened shortly before or after the final trip to the future and Death Phantom battle.)

Sailor Moon S *UP TO EPISODE 122*
April 1993 (Usagi 14, Grade 9)
December 1993 (Usagi 15, Grade 9)

S Movie
December 1993 (Usagi 15, Grade 9)

TIME RESET 

Sailor Moon S *EPISODES 123-127*
December 1992 (Usagi 14, getting ready to start Grade 9)

Sailor Moon Super S
April 1993 (Usagi 14, Grade 9)
January 1994 (Usagi 15, getting ready to start Grade 10)

Super S Movie
February 1994

Sailor Stars
April 1994 (Usagi 15, Grade 10)
October 1994 (Usagi 16, Grade 10)


Of course the Temporal reset business is still a giant cluster of you know what...  I mean it's not like the events of the first season were actually fully retconned because of the memories of not just Luna and Artemis but even Naru, and that dang puppet show proving that at least some of the Dark Kingdom stuff still had to have taken place.   So when did they take place?  Did events happen a year sooner?  Or did they end sooner with them being compressed into a couple weeks which could maybe fit in before the start of S?   Then what of the second reset...what happens to the Black Moon arc?  In theory could it have just not happened with Chibiusa being sent back to learn as a Senshi being her first trip back in the reset timeline?  *mind explodes*