In my humble opinion, 'can' is not noun but modal. It doesn't change its form.
Both 'tomorrow' and 'today' - given as supporting examples - are nouns.

1. Can you not go there?
2. Cannot you go there?

Which one's correct sentence? First one for me.

I also do not like hearing 'could' is the past simple of 'can'.
Grammarians mostly say the modals can not be conjugated after which they say 'could' is the past tense of 'can'. Interesting.