js 获取服务端时间，并实现时钟

本例子以vue语法伪代码书写：

1，获取服务端北京时间

getRealTime() {
            let that = this;
            var xhr = new XMLHttpRequest();  
            if( !xhr ){  
               xhr = new ActiveXObject("Microsoft.XMLHTTP");  
            }  
            xhr.open("HEAD",location.href,true);  
            xhr.onreadystatechange=function(){  
                if( xhr.readyState == 4 && xhr.status == 200 ){  
                    that.nowTime = xhr.getResponseHeader("Date");
                    that.changeTime();
                }  
            }
            xhr.send(null);
}

2，实现闹钟走动

changeTime() {
            let that = this;
            if(that.nowTime){
                setInterval(() => {
                   var t = new Date(that.nowTime).getTime();
                   t += 1000;
                   that.nowTime = new Date(t).pattern('yyyy-MM-dd HH:mm:ss');
                }, 1000);
            }
}

3,页面使用

<div>北京时间:{{nowTime}}</div>