[求助]如何实现倒计时?

<SCRIPT language="javascript">
var _t;
var _h = 0;
var _m = 1;
var _s = 0;
var _v = formatTime(_h + ':' + _m + ':' + _s);

function doTime() {
_s --;
if (_s == -1) {
_m --;
_s = 59;
}
if (_m == -1) {
_h --;
_m = 59;
}
var _b = ((_h == 0) && (_m == 0) && (_s == 0));
if (_b) {
_v = 'Welcome To IECN.NET';
clearTimeout(_t);
} else {
_v = formatTime(_h + ':' + _m + ':' + _s);
_t = setTimeout('doTime()', 1000);
}
document.all['oTime'].innerHTML = _v;
}

function formatTime(_time) {
return _time.replace(/\b(\w)\b/g, '0$1');
}

onload = function() {
document.all['oTime'].innerHTML = _v;
_t = setTimeout('doTime()', 1000);
}
</SCRIPT>