grammar check

Hi, here is a sentence that is really confusing me -

We had known each other for 3 years by April.

I think it's no problem in refer to the grammar used in it, but some say it sounds somewhat strange because of the length of time - "for 3 years" in the questioning sentence.

I'd appreciate if you could give your opinion about it.

[Casiopea]

Welcome.

The problem I see isn't the preposition "by". It's modality. Try adding the modals 'will' and 'would', like this,

By (this) April, we will have known each other for 3 years.
By (this) April, we would have known each other for 3 years. (we are not friends anymore)