Board Thread:General Discussion/@comment-26006155-20190820122909/@comment-26006155-20190826004740

Ambassadoe, we still don't have confirmation (nor any evidence that I can see) that the monsters are even using a different calendar.

They once lived on the surface with humans peacefully. And probabably adopted the same calendar, and had no reason to get ride of it down below.

Even if the monsters do use a difference calendar... the technological similaries between their version of the 2010's and our own is so similar that one might as well mirror the other.

Any case, all dates in Undertale use the Monster Calendar. So all dates in Undertale still have to follow the same logic.

And that still means that Asriel's birth in the ruins, the foundation of the Core/NewHome Complex, and Chara's arrival at the already-establish New Home still all happened in the same 201X decade.

Chara and Frisk did not arrive in the Underground three centuries apart.