How do GA and Yandex.Metrica determine the time spent on the site and the page in particular?

Question

JavaScript

Hello,
How do GA and Yandex.Metrica determine the time spent on the site and the page in particular?

Answer 1

The metric knows the hit time - page opening, for example.
And he knows the time, what time the second hit came.
Subtracted one from the other - it turned out the time on the page.
If there is no second hit, then refusal)

Answer 2

like this