JavaScript countdown function implementation code_time and date

/**
* Countdown
*
* @author WadeYu
* @date 2012-04-20 17:40
* @copyright boyaa.com
*/
var TimeCountDown = function( initTime ){
var day = 0;
var hour = 0;
var minute = 0;
var second = 0;
var timerId = 0;
var self = this;
var fixInitTime = function(){
initTime = parseInt(initTime);
if ( isNaN(initTime) || initTime < 0 ){
initTime = 0;
}
};
var fixZero = function(num){
return num < 10 ? ('0' num) : num;
};
var calc = function(){
showCallback.call(self, [fixZero(day), fixZero(hour), fixZero(minute), fixZero(second)]);
if ( day === 0 && hour === 0 && minute === 0 && second === 0){
self.stop();
} else {
if (hour === 0 && day > 0){
hour = 24;
day--;
}
if (minute === 0 && hour > 0){
minute = 60;
hour--;
}
if (second === 0 && minute > 0){
second = 59;
minute--;
} else {
second--;
}
timerId = window.setTimeout(function(){
calc();
}, 1000);
}
};
var showCallback = function(){
alert('Please set show callback functions!');
};
var init = function(){
fixInitTime();
if ( initTime > 0 ){
day = Math.floor( (initTime / (24*60*60)) );
initTime -= day * 24*60*60;
hour = Math.floor( initTime / (60*60) );
initTime -= hour * 60*60;
minute = Math.floor( initTime / 60 );
second = initTime - minute * 60;
}
};
this.start = function(){
calc();
};
this.stop = function(){
timerId && window.clearTimeout(timerId);
};
this.setShowCallback = function(fn){
typeof(fn) === 'function' ? (showCallback = fn) : '';
};
init();
};