Keep track of how much time is spent showing certain elements on the page

Question

So lets say we have 4 Divs (3 hidden, 1 visible), the user is able to toggle between them through javascript/jQuery.

I want to calculate time spent on each Div, and

Answer 1

At any point, you can record a a start/lap time in a variable with:

var start = new Date();

When you want to calculate the elapsed time, simply subtract the stored date from a new Date instance:

var elapsed = new Date() - start;

This will give you the elapsed time in milliseconds. Do additional math (division) to calculate seconds, minutes, etc.