My reply: Casiopea has already explained the meaning of a span of time:What is meant by "Sometimes we span, sometimes we don't"?
Our meaning is, :D in Ex1, the time "started in the Past and continues up to the Present", while our normal Present Perfect structures, such as Ex2, don't express such a span.Well , the action started in the Past and continues up to the Present. Past time (-en) is used to start the continuum and Present time (have see) is used to complete it. Time is used in this way to create a span of time, of which a start point and an end point is needed.
Ex1: I have seen him in the past week. (we span.)
Ex2: I have lived in Japan before. (we don't span.)