?是时区的问题么

<script type="text/javascript">
function countDown(endDate) {
 var now = new Date();
 var deadtime = document.getElementById(endDate);
 var deadline = new Date(deadtime.value);
 //本地时间与格林威治标准时间 (GMT) 的分钟差
 var timeDiff = now.getTimezoneOffset();
 //此处有些不解，如果把分钟差转化成毫秒应该是timeDiff*60*1000，但是这样返回的数据不正确！
 var leave = Math.abs(deadline.getTime() - now.getTime() + timeDiff*60);
 var minute = 1000 * 60;
 var hour = minute * 60;
 var day = hour * 24;
 var countDay = Math.floor(leave/day);
 //var countHour = Math.floor((leave - day*countDay)/hour);   两种计算思路
 var countHour = Math.floor(leave/hour - countDay*24);
 var countMinute = Math.floor(leave/minute) - countDay*24*60 - countHour*60;
 var countSecond = Math.floor(leave/1000) - countDay*24*60*60 - countHour*60*60 - countMinute*60;
 var outStr = "";
 if(deadline < now) {
  outStr = "距离"+deadtime.value+"已有"+countDay+"天"+countHour+"小时"+countMinute+"分"+countSecond+"秒";
 } else {
  outStr = "距离"+deadtime.value+"还差"+countDay+"天"+countHour+"小时"+countMinute+"分"+countSecond+"秒";
 }
 var showTime = document.getElementById("showTime");
 showTime.innerHTML = outStr;
}
</script>
<div style="border:2px solid "><label for="deadline">截止日期</label>
 <input type="text" value="2013-01-04" id="deadline" size="10"/><input type="button" value="确认" onclick="window.setInterval('countDown(\'deadline\')', 1000)" />
<p id="showTime" style="padding:5px 3px;margin:2px 1px;"></p>
</div>