pagevisibility API

<!DOCTYPE HTML>
 <html>
 <head>
      <script type="text/javascript">
           timer = 0;
           function onLoad(){
                document.addEventListener("visibilitychange",stateChanged);
                document.addEventListener("webkitvisibilitychange", stateChanged);
                document.addEventListener("msvisibilitychange", stateChanged);
           }
           function stateChanged(){
                console.log(document.webkitVisibilityState);
                if(document.hidden || document.webkitHidden || document.msHidden){
                     //new tab or window minimized
                     timer = new Date().getTime();
                }
                else {
                     alert('You were away for ' + (new Date().getTime()-timer)/1000+ ' seconds.')
                }
           }
      </script>
 </head>
 <body onLoad="onLoad()">
 </body>
 </html>
Snippets

pagevisibility APIedit
