"It's afternoon. It's the afternoon." both are accepted. Shouldn't the latter be ettermiddagen? Why is that suggested as a possible option?
Because 'it's afternoon' is a direct translation, which would be uncommon. So the difference is that Norwegian tends to use the indefinite form for this noun, while English tends to use the definite form.
(yes--you'll find lots of words that do this... In both sensical and nonsensical ways. I think my favorite is the word for vegetable, grønnsaker, literally meaning "green stuff")
I think two of my favorites are:
esophagus - spiserør - "eating pipe/tube"
constellations - stjernebilder - "star pictures"
These are the best kind of Norwegian words and make me wonder why we had to have such complicated and nonsensical ones in English.