Board Thread:General Discussion/@comment-26006155-20190820122909/@comment-26006155-20190824184508

You're quite right, that the monsters couldn't use the sky or the seasons to tell time in the underground. That without some outside frame of reference, telling time would be almost impossible.

Which is the very reason the monsters would have adopted the human calendar, given the chance.

There is one window the monsters have to the outside world: Human Garbage. And since Gaster has been forgotten, the only source of modern technology that Undyne remembers comes from the study of all the human garbage falling down into the dump in Waterfall.

The intro to the game sets Chara's Fall in 201X. And Chara herself confirms that she arrived at New Home (which was then the Capital, as the migration from Home had already happened) at "The End of 201X". We definitely have a starting point for Chara's adventures in the Underground.

But how can it possibly be centuries in the future, after that time?

Gaster's otherworldy technology is limited to The Core, which houses The True Lab, and the secret lab behind Sans' house. Outside of these areas, the technology show is surprisingly modern.

Toriel used a modern handheld camcorder to record her discussion with Asgore about having a child. Napstablook is all about 8bit tunes and CDs and Napster-era sound mixing technology. And Snowdin looks like a modern town with electric lightly, and only a touch of super tech with the Linked Boxes and it's fast-transit tunnel.

If it was really 2400 A.D., the monsters would have had 4 centuries to study all advances in human technology made in that time. The entire Underground would look like The Core by then. Yet while Undyne is confused whether anime is real or not, Toriel knows that it isn't real. The humans of this world don't have really cool stuff like Real Transformers yet. Only Gaster has that level beyond-surface tech, because he's from another world entirely.

You say that the monsters and humans must be using different calendars. But this is an asumption, not a fact. And I see plenty of evidence that questions it.