Also. I haven't seen pretty much anyone mention it at all anywhere in the community, but I'm completely certain that year dates are going to have extreme relevance. I don't have an actual theory about it, I just know that it's going to matter.
First of all, what brought this thought to me was the simple and obvious fact that Spamton's favorite year is 1997. Which he emphasizes greatly. Naturally, his favorite year would also likely be when he was most successful, or rather, when he was a big shot. That also may imply that the year in the Light World is also 1997. Which is consistent with what we see. Box TV's that still use antennae, old-fashioned computers, an old-fashioned phone, the clothes characters wear (primarily Susie), and the school itself doesn't seem modern.
Going back to Undertale, there is also great focus on the years in question. In the intro it says that, much after the Monster Vs. Human war, the year is 201X. Which is where we see the scene of Chara, not Frisk, falling into the Underground, In Toriel's home, the calendar is noted as being old, and comes from the beginning of 201X. In New Home, the calendar here is also old, and is from the end of 201X, with a date circled on it. It's also said that the Monster's had gone to the very back of the cavern first, which is where the Ruins are, to be as far from the Barrier and humanity as possible. But eventually they started to make the underground their home, going from there all the way back to the front, where the Barrier is, building along the way. Apparently it took them only about a year to do all of that. And during that timeframe, Chara fell into the underground.
This also makes me wonder how long the gap actually was, though. Chara would've needed to have enter the underground near the end of 201X as well, meaning their death was more than likely in the following year. If the gap between then and Frisk's arrival was really as big as most people believed, the actual game would be taking place in the very far future. Most likely, I'd guess Chara actually died in 2011 or 2012, and because there were 6 humans between that and Frisk, possibly 1 year for each human, meaning Frisk may have arrived in 2017 or 2018. But that's another topic.
On Twitter, Toby made two tweets a while back at the start of 2020. Which didn't actually give any real hints. But I wanted to mention it.
"It became the year 2020.
The era of 201X is now over..."
Followed by a Tweet that said:
"Meanwhile, in 199X"
Then he posts this image.
I doubt any of that has actually relevant meaning, honestly. Probably just a callback to Undertale because the year 201X was relevant, and a joke in the next tweet. But at the very least it once again brings up the year 201X as a relevant date.
So, Deltarune takes place in likely 1997, or at the very least before 2000. Whereas Undertale all takes place in the 2010's, with the Chara backstory being in probably 2011 or 2012, with the main events of the game being much later into the 2010's. It's also possible that the game's main events actually do take place in 2015, and maybe some of the humans in between Chara and Frisk were much closer together than previously thought.
Can any theories be derived from all this?