Während zweier Jahre kannte ich sie.

English Translation

I have known her for two years.

It doesn’t seem to me a proper translation. Can someone please explain?

This is a weird sounding sentence indeed. I think the author misunderstood the meaning of the english sentence.
It means something like “I knew her for two years (and after those two years I didn’t anymore)”.

The usual translation for what the english sentence wants to say (i.e. I met her two years ago, and I have known her ever since) is:

Ich kenne sie seit zwei Jahren.