Translation:It's not eight o'clock now.
Instead of 「じゃない」, it might be better to use 「ではない」; I usually thought「じゃない」 is usually (just) used after adjectives (studied adjectives [and nothing else] for WAAAAY TOOO LONG... apparently), but it is just as good I'm sure (but what would I know, 3 years in study and never studied 「です」 long enough to know 「じゃない」is also casual...) Although, I agree with you; 「ではありません」sounds a bit too formal to be natural.
ではありません is the negative of です. So just like です will often correspond to "am/is/are/etc." in English, ではありません would be "am not/is not/are not/etc." So for example, 八時です would be "It's 8:00," while 八時ではありません would be "It isn't 8:00."
ありません by itself (no では) is the negation of あります, which is the word for "to exist/to have" for inanimate objects. So ありません is "doesn't exist/don't have" for inanimate objects. For example, if you went to a restaurant and asked if they had a hamburger, they might respond ありません if they don't sell hamburgers.
Sorry this may be dated but if others are looking for a reply to this, the 'は' between '今' and '八' is because 'は' is a subject particle and whatever comes before it is usually the main topic or subject of the sentence. Since '今' comes before 'は' in the sentence, we make it clear to the reader or listener that we are talking about right now (since '今' is equivalent to saying 'now' in english). You can say '八時です' meaning 'It is eight o'clock' (implying it is eight o'clock now) but to make it more assured that we are talking about now, we add '今は' to the beginning of the sentence!
I am very new to the language so this may not be entirely correct, but I hope this helps anyone who is looking to understand more about particles. There is a helpful guide to particles (plus lots more) on youtube from japanesepod101 if you are looking for more information!
"Now it's not eight o'clock" is an unnatural English sentence. Now should be at the end, although that would be superfluous. Perhaps the proper option would be "It's not eight o'clock [right now]" with the  being optional/uncommon/informal. Also, Duolingo shows you your typos at the bottom of the screen.
You've got to think, someone who knows no Japanese would be the very person who is taking the lessons. Everyone's got to start somewhere, and not everyone stays at the same pace. Some people need to come check out what certain things mean in the sentence and others already know or assume. You can change your default format from the building block words to free write.