你的这个问题是涉及强调句式“用It is还是用It was”的问题。这与not...until...句型没有直接的联系。

我们可以根据原句时态，来确定用It is还是用It was。如果原句时态属于现在时间范畴（包括一般现在时，现在进行式，现在完成时，一般将来时等），则用It is …；如果原句时态属于过去时间范畴（包括一般过去时，过去进行时，过去完成时，过去将来时等），则用It was … 例如：

It is at ten o’clock that the meeting will take place.

What is it that you want me to do now?

It was not until yesterday that I heard the news.

Was it John that broke the window this morning?

请问not...until…的强调句式可以用于一般现在时吗？——完全可以。你问这个问题，可能的一个原因是，你碰到的not...until...句子，都是用了过去时间范畴的时态，以至于你产生这个疑问。没关系！

其实，not...until...句子用一般现在时或将来时并不鲜见。例如：

You won’t succeed until you try three times.

It is not until you try three times that you will succeed.

此外，情态动词还可用于It be中：

It might have been John that bought a new book for Mary yesterday.

It may be next week that she leaves for Tokyo.

It mast have been a thief that broke in the house.

It can’t be Tom that has damaged the desk.

It will not be you that will have to take the blame for this.