It is useful to distinguish two uses of the Present Perfect Tense.
Unspecified Time Before Now
Duration From the Past Until Now (Non-Continuous Verbs)
Use 1 Unspecified Time Before Now
We use the Present Perfect to say that an action happened at an unspecified time before now.
The exact time is not important.
Examples:
I have seen that movie twenty times.
I think I have met him once before.
There have been many earthquakes in California.
People have travelled to the Moon.
People have not travelled to Mars.
Have you read the book yet?
Nobody has ever climbed that mountain.
What is an unspecified time before?
The concept of "unspecified time" can be very confusing to English learners.
It is best to associate Present Perfect with the following topics: