I personally agree with the "Frisk is from the future" explanation. It does seem rather unlikely that 10 years would be enough for 6 more humans to fall and die, and that no one knows what the former queen would look like. Plus, maybe it was an old calendar that Asgore couldn't bear to throw away.

I'd like to elaborate more, but I can't be bothered to. I'm not [i]that[/i] big a fan of Undertale. :P