questions - Should I use "after" or "until" when saying when a place will be closed?

What should I write?

What time does it [a restaurant] close?

Not after/until midnight. You have plenty of time.

Should I use after or until in the first sentence of the reply?

Here's the context: I've just asked for directions to the restaurant. After I've been told the directions, I say:

Thanks! What time does it close?

The answer will be:

Not after/until midnight. You have plenty of time.

(This was taken from an English textbook.)

Answer

So you have two choices:

Not after midnight.
Not until midnight.

After: later than something; following something in time
Until : up to the point in time or the event mentioned

If you say "Not after", you are saying that the restaurant doesn't close later than midnight. It could close any time before midnight, so you haven't answered the question "What time does the restaurant close?". You have said when it doesn't close.

If you say "Not until", you are saying that the restaurant doesn't close up to the time of midnight, then at midnight, it closes. That does answer the question "What time does the restaurant close?".