"It's not six o'clock now."
All of them are used in the end of sentence in polite language. This are my thoughts on them:
ではありません is the most correct one, esp. for written language (unless you're mimicking speech), but it is often replaced with shorter じゃありません in spoken language. Both are more correct than じゃないです and ではないです, which sound like (relatively) young or uneducated people trying their best to be polite by adding です to every sentence.
Sometimes learners think they hear じゃないです when they just couldn't catch the ん in じゃないんです.
Why is it, asking for current time require no は, (今、何時ですか) but saying current time it isnt, requires it? (今は六時ではありません) does it work without the は? (今六時ではありません). And likewise, would it work with the は in first example (今は何時ですか)?
Maybe I'm just not good with understand different instance... Please don't hurt me internet. XD
It is true that the usage of the は (ha) particle after 今 (ima) is usually optional. The same should be true for this instance. However, Duolingo will usually not accept negative sentences that lack the は particle. In later lessons, other users will explain that Duolingo does this because the は particle plays a much more significant role of "emphasizing a topic" when used in negative sentences.
P.s. Please bear in mind that I'm just a newbie to the Japanese language. So, if I got anything wrong, feel free to correct me. Ty.