The choice between present perfect and simple past reflects the time we are talking about. We use present perfect to talk about things in time that has not finished, and past simple to talk about things in time that has finished. Here is an example: Have you been to New York? Yes, I've been to New York- I was there for a month in 1990. I have been there refers to an unfinished time: your life. I was there refers to a time that has finished: one month in 1990. It is not correct to say that using present perfect means that you don't know exactly when something happened.

Sure. When you say something like: I've been there, you mean that you have been to somewhere but you dont know exactly when. when you say: I was there you must say exactly when, like a "I was in the gym today morning" or yesterday.