Board Thread:General Discussion/@comment-26006155-20190617222636/@comment-26006155-20190714222606

Ok, I need to hunt through the game for clues to how old various locations are.

The Librarby says that the monsters returned to New Home when they were no longer afraid of humans. And during Undertale itself the monsters do seem a bit Gung-ho about the destruction of Humanity. Feeling certain that they're the ones who are going to win.

I don't see how the arrival of one little human child could change this.

... but Gaster's Super Technology certainly would.

So I'm pegging the change at when Gaster (and Sans) arrived in the Underground/Ruins.

The timeline where Gaster built the Core, researched Determination (or already knew about it and made the blueprints, did he die just before he could build the actual machine?) and then died before Chara arrived still makes sense then.

And the clue that The Buttercup Adventure could have only happened in 202X still fits, too.