SO I'm almost done with chapter 5 (I think?), but I have a really silly theory
(under line for spoilers)
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SO I'm almost done with chapter 5 (I think?), but I have a really silly theory
(under line for spoilers)

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Haven't seen anybody talk about this, but the code in Asgore's shop is clearly a date.
September 15 is the day Undertale came out, of course, but why 199X? What happened that year? (Asgore and Toriel's wedding, maybe?)
The third code might be 5747 (like I sent in the ARG, for which I never did receive a reply email...), because [1] it's K-R-I-S (Kris, Ralsei, I, and Susie) on a flip-phone keypad, [2] the Mike room is just his name on a flip-phone keypad, [3] Kris deleted all their contacts, [4] phones are significant in this series, [5] it'd be the "delta rune" code in the "pine tree" (Dess guitar) & "sheriff badge" (Undyne) trio, [6] it's a perfectly fitting four digits, and [7] it'd be neat if Kris just had it all along. Even if it's complete bollocks, it's a fair guess.