Javascript snippet to time how long user spent on page

time_user.html

<script>
  // time how long user spent on page
  // following this example - http://stackoverflow.com/a/16046028
  var timeStart = new Date().getTime(); // time when user arrives
  window.onbeforeunload = function(){
    var timeEnd = new Date().getTime(); // time when user leaves
    var timeDif = parseInt(timeEnd - timeStart, 10)/60000; // calculate difference in minutes
    amplitude.logEvent('User Left Page', {'time_spent_minutes': timeDif});
};
</script>