#imCell_5 { display: block; position: relative; overflow: hidden; width: 904px; min-height: 543px; border-top: 5px solid transparent; border-bottom: 5px solid transparent; border-left: 5px solid transparent; border-right: 5px solid transparent;}
#imCellStyle_5 { position: relative; width: 804px; min-height: 443px; padding: 50px 50px 50px 50px; text-align: center; border-top: 0; border-bottom: 0; border-left: 0; border-right: 0; z-index: 2;}
#imCellStyleGraphics_5 { position: absolute; top: 0; bottom: 0; left: 0; right: 0; background-color: transparent;}
#imCell_7 { display: block; position: relative; overflow: hidden; width: 453px; min-height: 356px; border-top: 3px solid transparent; border-bottom: 3px solid transparent; border-left: 3px solid transparent; border-right: 3px solid transparent;}
#imCellStyle_7 { position: relative; width: 447px; min-height: 36px; padding: 3px 3px 317px 3px; text-align: center; border-top: 0; border-bottom: 0; border-left: 0; border-right: 0; z-index: 2;}
#imCellStyleGraphics_7 { position: absolute; top: 0; bottom: 0; left: 0; right: 0; background-color: transparent;}
#imCell_8 { display: block; position: relative; overflow: hidden; width: 449px; min-height: 356px; border-top: 3px solid transparent; border-bottom: 3px solid transparent; border-left: 3px solid transparent; border-right: 3px solid transparent;}
#imCellStyle_8 { position: relative; width: 443px; min-height: 350px; padding: 3px 3px 3px 3px; text-align: center; border-top: 0; border-bottom: 0; border-left: 0; border-right: 0; z-index: 2;}
#imCellStyleGraphics_8 { position: absolute; top: 0; bottom: 0; left: 0; right: 0; background-color: transparent;}
<html>
 <body>
 <div id="display"></div>
 </body>
 <script>
 var disp = document.getElementById("display");
 //getElementById("display");
 var time = 180; //secondi iniziali
 var timer = null;
 function cDown()
 {
 var h = Math.floor(time / 3600);
 var m = Math.floor(time / 60);
 var s = time % 60;
 var timeStr = (h<10 ? "0"+h : h) +":" +(m<10 ? "0"+m : m)+":" +(s<10 ? "0"+s : s) ;
 disp.innerHTML = timeStr;
 time--;
 if(time==0) window.clearInterval(timer);
 }
 
timer = window.setInterval("cDown()",1000);
 //.setInterval("cDown()",1000);
 </script>
 </html>
 