[susan53]

69. Zach has known Mike for about a year.
Use the present perfect when the exact time of a past repeated
action isn’t important.

Again this is the traditional (though inaccurate) answer.

All perfect verbs sequence events as happening before an implicit or explicit reference point. In the case of the present perfect, if the reference point is unstated, it is always the moment of speaking/writing - ie the "present" moment for the speaker/writer.

In other cases, the reference point will be stated explicitly- eg : When you've fried the onions, add the tomatoes. Here, the reference point is the moment of adding the tomatoes. The use of the present perfect tells us that "frying the onions" has to happen before that.

And if you look at all the other examples of perfect verbs, the same is always true. There is a reference point which may be present (70 - again, the moment of speaking), past (71 - the moment they played at the club together) or future (72 - the moment when he'll get his check) and the perfect verb sequences the event which it describes as happening at some time before the reference point.