js获取服务器时间的方法

function getServerTime(){
    return new Promise((resolve,reject)=>{
        let 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 ){
                resolve(xhr.getResponseHeader("Date"))
            }  
        }  
        xhr.send(null);
    })
}

new Date($.ajax({type:"HEAD",async:false}).getResponseHeader('Date'));