Instrumantal is used in sentences type "noun IS noun", i.e. that something is a member of some type of objects, like f.ex. "Pies jest zwierzęciem" [A dog is an animal].
Here you have a different statement: "something exists and has a property", it is a sample of very short sentence type "verb adjective noun" which serves to describe a state or circumstances - and translates into English as simple as to "pronoun verb adjective noun" - A pronoun is obligatory in English, but it is not in Polish.
- Jest późny wieczór (nominative case) [It is late in the evening]
- Jest wczesny ranek (nom.case) [It is early morning]
- Jest ładna pogoda [It is a nice weather]
Attention, it is something different than sentence type "noun IS adjective"! In the contrary, that structure serves to attribute a property to a noun:
- Wieczór (nominative case) jest późny [The evening is late (already)]
- Dziewczyna jest ładna [The girl is pretty]
- Jabłko jest czerwone [The apple is red]
Because "time" is exception? ( there must be better/wider explanation, but this works for hour, part of the day, day/night, day of week, month, season, year, century, millennium)
noc is feminine. It is kind of exception as there are not many -c ending feminine words, but there are many feminine nouns that end with "softer" consonants (s,sz, ś, c,ć, cz ż ź dż dź ń), I always say that you should toss the coin or check dictionary for them. (ść ending is more likely to be feminine noun)
why not "to ciemna noc". All exercises prior to this translate "it is" to "to", not simply "jest".
could I use "to"?
Yes, you could.
This is a nice sentence teaching an interesting nuance. Yes, you can understand the English sentence simply as "[It/This/That] is a dark night", constructed identically as let's say "[It/This/That] is a small dog", and then you translate it as "To [jest/] ciemna noc".
However, the main Polish sentence is constructed differently. "Jest ciemna noc" means that right now it is a dark night. It's similar in construction to "It is raining" ("Pada deszcz"), a sentence which (at least in Polish) doesn't have any grammatical subject, no real 'it' to translate - not even a dummy pronoun.