$start = Date.now()

$time_unfocus = 0;
$time_focus = 0;
$time_checkout = 0;
$time_checkin = 0;
$time_total = 0;
$tabswitches = 0;

$(window).focus(function() {
  $tabswitches += 1;
  $time_checkin = Date.now();
  if($time_checkin == $start){
    $time_checkin=0;
    $tabswitches -= 1;
  } else{
    $time_unfocus = ($time_checkin - $time_checkout)/1000;
    $time_focus = ($time_checkin - $start - $time_unfocus)/1000;
    $time_total = ($time_focus + $time_unfocus);
  }
});
$(window).blur(function() {
  $time_checkout = Date.now();
});