Time Spent by user on a web page + Iframe

I want to measure the time spent by a user on web pages. It's doesn't include the time when a user navigates away from the browser.

I found few open source libraries like Timejs library which can measure the time spent by a user on a web page, but when a user is watching a Youtube video on the page, these libraries don't measure that time.

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1. Is there a way to handle this case or any other library which handles this case?
2. Actually, these libraries fail to calculate the time spent by the user inside an iFrame.What is the best way to know when a user is active in an IFrame?
3. How does Google/AppNexus/Motomo calculate interaction time? Is there a common design/architecture that they follow?

PS: I am trying to integrate the library with my react App. If there exists any similar method to solve this problem in React. Please let me know in comments/answers.

Thanks in advance

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