track page view time

gistfile1.js

// get current time when page is first run
// (the + prefix is magic that will cast it into a timestamp)
var startTimestamp = +new Date();

window.addEventListener('beforeunload', function () {
    // get current time when the beforeunload event is fired
    var endTimestamp = +new Date();
    
    // calculate deltas
    var deltaTimestamp = endTimestamp - startTimestamp;
    
    // convert milliseconds -> seconds
    var deltaSeconds = deltaTimestamp / 1000;
    
    // log to console
    console.log('You stayed on this page for %d seconds. (%d milliseconds)', deltaSeconds, deltaTimestamp);
});