把这段code的秒钟、分钟和时钟的个位数变成两位数

[maiko2010]

请问下面这段code要如何才能显示把秒钟、分钟和时钟的个位数变成两位数？例如：0:0:0变成00:00:00

或者变成国际时间，
或者加上AM/PM。

谢谢！

<script>
var secs = 300;
function countdown() {
if (secs <= 0)
{
alert("您学习时间已完毕了")
window.open('www.yahoo.com.tw');
window.opener = null;
window.close();

}
else
{
var hh = Math.floor(secs/3600);
var mm = Math.floor("0" + (secs%3600) / 60);
var ss = ("0" + secs % 60).slice(-2);

document.getElementById("num").innerHTML =hh+":" + mm + ":" + ss;
secs--;

}
}
window.setInterval("countdown()",1000);
countdown();

</script>


另外为什么要少用onload="clock()"？
谢谢！

[sundotLei]

<script>  
var secs = 300; 
function countdown() { 
if (secs <= 0) 
    {  
            alert("您学习时间已完毕了")  
    window.open('www.yahoo.com.tw'); 
    window.opener = null; 
            window.close(); 

      }          
  else 
{ 
    var hh = Math.floor(secs/3600); 
    var mm = Math.floor("0" + (secs%3600) / 60); 
var ss = ("0" + secs % 60).slice(-2); 
var time=hh+":" + mm + ":" + ss;
time=time.replace(/\b(\d)\b/g,"0$1");
document.getElementById("num").innerHTML =time;
secs--; 

} 
} 
window.setInterval("countdown()",1000); 
countdown(); 

  </script> 

[sundotLei]

修改过的部分
var time=hh+":" + mm + ":" + ss;
time=time.replace(/\b(\d)\b/g,"0$1");
document.getElementById("num").innerHTML =time;

[maiko2010]

试过了不行。