I've been meaning to find this out for awhile and I don't think you've ever stated it yet. But what's your cannon for the human world above? is it an exact replica of our own just with monsters hidden beneath a single mountain. Or is it a Pangea continent with modern tech but royal leadership?
In this comic, the surface during the time Chara fell in 201X would match the real world (between 2010-2017).
In the comic’s present time of 2112, it’s still our world, only about a hundred years into the future. Because our story focuses on Frisk and Chara’s interactions in the Underground, the state of this future world will not be explained in detail since it is not relevant. What we can say is that there is advanced technology but no futuristic royal monarchy.