function StartTime()
{
var today=new Date()
var h=today.getHours()
var m=today.getMinutes()
var s=today.getSeconds()
// add a zero in front of numbers<10
m=checkTime(m)
s=checkTime(s)
document.getElementById('txt').innerHTML=h+":"+m+":"+s
t=setTimeout('startTime()',500)//500是半秒执行一次你半小时应该是。。。。
}

------解决方案--------------------

探讨

Calendar now = Calendar.getInstance();
Stirng hour = Integer.toString(now.get(now.HOUR_OF_DAY));
if(hour.length()==1){
hour="0"+hour;
}
Stirng miniter = Integer.toString(now.get(now.MINUTE));
if(miniter.equals("30")){
System.out.println(时间是:+hour+":"+miniter)
}

没要求输出是时间格式吧


------解决方案--------------------
Java code

<%
for(int i=0;i<=24;i++){
String h_str="";
    if(i<10){
        h_str = "0"+i;
    }else{
       h_str = String.valueOf(i);
    }
    out.println(h_str+":00");
    out.println("<br/>");
    if(i!=24){
    out.println(h_str+":30");
    out.println("<br/>");
    }
}
%>