Learn Hardcore Chinese: bānjiāgōngsī bùshì jīntiān lái, jiùshì míngtiān lái. - The moving company will come either today or tomorrow.

Breakdown of bānjiāgōngsī bùshì jīntiān lái, jiùshì míngtiān lái.

What does the pattern 不是……就是…… mean in this sentence?

It means either...or....

So:

This pattern presents two possibilities, and the speaker believes one of them is true.

Does 不是 here literally mean is not?

Not exactly. In this sentence, 不是 is part of the fixed pattern 不是A，就是B.

So here it does not simply mean a standalone not. Instead, the whole structure means:

So 不是今天来，就是明天来 means:

Why is 就是 used here? Does it mean is?

In this sentence, 就是 does not mean the basic to be meaning that beginners often learn first.

Here, 就是 is the second half of the pattern 不是……就是…….

So:

So you should understand 就是 here as part of the grammar pattern, not as a simple is.