gistfile1.js

(function() {
  var lastSeen  = (new Date()).getTime();    
  var engagedTime = 0;
  var idleTimeout = 5000;

  $(document).ready(function() {
    $(window).on('focus click scroll mousemove touchstart touchend touchcancel touchleave touchmove', function() {
      lastSeen = (new Date()).getTime();    
    });
  });

  setInterval(function() {
    var timeSinceLastEvent = (new Date()).getTime() - lastSeen;
    if (timeSinceLastEvent < idleTimeout) {
      engagedTime += 1;
    }
  }, 1000);
  
  window.engagedTime = function() {
    return engagedTime;      
  }
}).call(this);