Author's Notes:

There you have it.

I deliberately wrote the story to set up both the "They go back" and the "They don't go back" endings, to keep people from figuring it out through literary analysis. It's too boring to figure out a mystery early, then just slog through the story to see it resolve. And 350 is definitely a mystery in that respect, with the seemingly contradictory evidence.

I (hopefully) did a good job at misleading the readership into thinking that the two outcomes were mutually exclusive. But I have two ponies; I could do both. And for the curious, Three Hundred and Fifty is single-timeline inviolate. Everything seen in the fic is set in stone, nothing Twilight can do will alter what she or Sweetie experienced in the future.

And I do like the idea of an optimistic future for once. Despite the changes, it's still Equestria, and they are still ponies. The first thing that happens when Twilight gets to the future is that Sweetie's life is saved by a random faceless, nameless pony.

It should be obvious in hindsight why I skipped past years like I did. Sweetie Belle needed to be old enough to live on her own (well, with Spike at least). There'd be no way Sweetie could rationally choose to stay behind as a filly (or for Twilight to let her). The original outline called for 15 years passing, but I just ran out of material. Oh well.

On Celestia's lie: This was the answer to the question "Why can't Twilight ask a princess or look in a history book to find out whether she made it back?" that came up during outlining. It would be a perfectly logical thing for Twilight to do, and also destroy the story. This was the best solution I could come up with, with my best attempt at keeping it in character. Also notice that every piece of bad news she delivers is immediately followed by good news, to get Twilight's hopes of the future up.

Cadance: As far as 350's headcanon is concerned, despite being an alicorn she was still mortal, and died long ago with everypony else.

Princess Twilight: The joys of starting a fic mid-season. Making her an alicorn wouldn't be THAT difficult. I gave her wings anyways, after all. Introducing the amulets (and therefore the weather thing) properly would be trickier. There's no reason the Twileycorn couldn't happen after she gets back, although wings on her grave might tip unicorn-Twilight off that something's up. Or maybe the Twileycorn was immortal and hiding behind a bush for ten years.

The mystery:
I took great care to make everything in the story consistent with the outcome. Here's some off the top of my head:
Catching Up With the Sun: Yes, the first sign was right at the start of chapter three. Princess Celestia had been drinking tea for her nerves for over a decade. Not millennia. Not centuries. One decade. Like she was anticipating something. Twilight's favorite blend, too. Sure, she'd figure that Twilight was coming either way, but how would she know when? Unless somepony told her.
Old Friendships: The graves, of course. Twilight really is buried there. And the Sweetie Belle statue to show that she doesn't go back.
A View Like None Other: Don't you think that Luna is just a little TOO informal with Twilight? Not using her last name, for instance? Like Luna's talked with Twilight. Repeatedly. Over many many years.
One Last Reunion: Nothing Spike says is technically false. He doesn't want to lie to Twilight, and Celestia cowed him into going with her plan, so he just carefully picks his wording. Kinda suspiciously, in my opinion.

There are a few questions that I intentionally leave open. Like what the Main 5 got up to while Twilight was gone. Or how the rest of Sweetie Belle's life went. Or how and when Twilight eventually died. Or whether Twilight ever did forgive Princess Celestia. Make your own answers to those questions.

Thanks again for reading a fic that became a heck of a lot more popular than I'd expected. Seriously I was anticipating maybe a hundred views of this thing. But I had a fun time reading the comments (yes I read every one of them) and looking at speculation. Makes me feel all fuzzy inside.

Oh, and for the time-loop-challenged, I have provided a handy timeline of the story, as seen from outside the loop:

Year -25(approx): Twilight Sparkle & other 5 born
Year -12(approx): Sweetie Belle born
Year 0: Prologue, Twilight and Sweetie enter the apparatus to go to the future
Year 10: Epilogue, Twilight returns
(sometime in here Twilight gives Princess Luna the answer on whether she forgives Celestia, even if Luna won't understand for a while)
Year 45(approx): Twilight Sparkle & other 5 die. Assuming lifespan of 70 years
Year 310(approx): Griffon Civil War ends. Princess Celestia, anticipating Twilight's return, crafts her plan to keep her in the future. Princess Luna, disagreeing, argues with and separates from Celestia
Year 338(approx): Apple Crisp & Spot Light born
Year 340(or so): First moon colonization
Year 350: Twilight Sparkle and Sweetie Belle exit the apparatus, where the Nexus is destroyed
Year 351: Sweetie Belle gets her cutie mark
Year 355: Much ado with the moon and fire
Year 360: Spike returns, Twilight rebuilds the apparatus then leaves for the past. Luna gives Celestia her answer
Year 408(approx): Sweetie Belle, Apple Crisp, Spot Light die