I've just come from a closed thread, without seeing a clear answer (since John has died). I don't know if I can discuss this question here.
- It's been over 20 years since I've lived on that street*
means it's over 20 years since I left that street.
A better version of 1 is:
- It's been over 20 years since I left that street.
- I've known her since I've lived on this street.
means I still live on this street.
1 and 3 both contain "since + present perfect", but the structure of the two sentences are different, which leads to different interpretations of the two since clauses:
In 1, since clause refers to a completion, while in 2, the since clause refers to continuousness.
A better version of 3 is:
- I've known her since I've been living on this street.
Do you think my understanding is correct?
*The original sentence is "It's been over 20 years since John has lived in that country". I rephrased the sentence.