Time Spent on a Document

We have a website which we are using to support our customer. They can login to the application search for the issues they are facing, and based on their search we provide them documents which can go through. We want to implement we analytics on this website with two main objectives. 1. Get the list of documents which the user has viewed based on the his search results. 2. Time spent by the user on a particular document. I checked the current standard and seems that getting the document list which the user has visited is relatively easy. But to calculate the time spent on the document is tricky. The way to calculate time spent on a page is to find the difference between two consecutive hits. But this approach will not calculate the correct time spent on document in some scenarios like: 1. When user is viewing multiple tabs for viewing the document and toggling between the tabs. 2. When user opens a documents reads it and leaves the site from that page. Both the parameters , List of documents and times spent on a document are important ,as they will help us usefulness of our documents and identify the documents which needs to be updated. So we need a robust method to calculate the both.