No, they're adverbs, not nouns -- in English and German both.
They tell us when something happened: yesterday, we went to the park. You say "on yesterday, we", not "on the yesterday". Even this little snippet expands to "on yesterday, on today, on tomorrow".
Now and again they get converted to nouns: "all our yesterdays have lighted fools the way to dusty death", but you can tell because you can use an article with them: "the yesterdays have lighted" -- "die Gestern beleuchtet" (I think...)
(Thanks for asking, it made wonder too, and figure it out)
So Morgen is a noun and morgen is an adverb that makes bout as much sense as Sie or sie they or she lets hope its not something important we say because if theyre pronounced the same and you are in a crowd of people and you tell a wall eyed friend to tell you something and you are he in a crowd which is they and the something is , morgen meaning good morning casually , and you or they take it as tomorrow , and the tell the friend to let you know to duck or be shot half the they might you dont and none of it makes any sense why in the world intelligent people would ever give any word more than one meaning let alone 4 or 5 is beyond me but even funnier is all you people that act like this correct and if you had knowledge , none of your knowledge matters when it looks screwy on paper and in real conversation is an accident bound to happen from misinterpretation .
In German the first letter of a noun is capitalized. So these are actually two different words:
(der) Morgen = (the) morning [NOUN]
morgen = tomorrow [ADVERB]