Sync server time to client browser with JS. Implement follow Network Time Protocol.

server_time_sync.js

// Thanks http://stackoverflow.com/questions/1638337/the-best-way-to-synchronize-client-side-javascript-clock-with-server-date

var serverTimeOffset = false;
function getServerTime(callback) {
    if (serverTimeOffset === false) {

        var scripts = document.getElementsByTagName("script"),
            URL = scripts[scripts.length - 1].src;

        var clientTimestamp = Date.parse(new Date().toUTCString());
        var xmlhttp = new XMLHttpRequest();
        xmlhttp.open("HEAD", URL + "?noCache=" + Date.now(), true);
        xmlhttp.onload = function(){
            if (xmlhttp.readyState === 4) {
                if (xmlhttp.status === 200) {

                    var serverDateStr = xmlhttp.getResponseHeader('Date');
                    var serverTimestamp = Date.parse(new Date(Date.parse(serverDateStr)).toUTCString());

                    var serverClientRequestDiffTime = serverTimestamp - clientTimestamp;
                    var nowTimeStamp  = Date.parse(new Date().toUTCString());

                    var serverClientResponseDiffTime = nowTimeStamp - serverTimestamp;
                    var responseTime = (serverClientRequestDiffTime - nowTimeStamp + clientTimestamp - serverClientResponseDiffTime )/2;

                    serverTimeOffset = (serverClientResponseDiffTime - responseTime);

                    var date = new Date();

                    date.setTime(date.getTime() + serverTimeOffset);

                    callback.call(null, date);
                } else {
                    console.error(xmlhttp.statusText);
                }
            }
        };
        xmlhttp.send(null);
        
    } else {
        var date = new Date();

        date.setTime(date.getTime() + serverTimeOffset);

        callback.call(null, date);
    }
}

var clientTimeStamp = Math.floor(getServerTime() / 1000);

getServerTime(function(time){
console.log(time)
})

I run it on Chrome and got the following warning Synchronous XMLHttpRequest on the main thread is deprecated because of its detrimental effects to the end user's experience. For more help, check https://xhr.spec.whatwg.org/.
Have you noticed that warning?

I am getting the same error. Synchronous XMLHttpRequest on the main thread is deprecated because of its detrimental effects to the end user's experience. For more help, check https://xhr.spec.whatwg.org/.

@sela @muskkir I just update code. This error cause from last update spec that force all xmlHttpRequest must call as async.

I just update code with callback. You can implement it as promise or reactive if you need.

This maths are wrong but I don't know where:

serverClientRequestDiffTime = serverTimestamp - clientTimestamp;
serverClientResponseDiffTime = nowTimeStamp - serverTimestamp;
responseTime = (serverClientRequestDiffTime - nowTimeStamp + clientTimestamp - serverClientResponseDiffTime )/2;
serverTimeOffset = (serverClientResponseDiffTime - responseTime);
serverTimeOffset = nowTimeStamp - serverTimestamp - (serverClientRequestDiffTime - nowTimeStamp + clientTimestamp - serverClientResponseDiffTime )/2
serverTimeOffset = nowTimeStamp - serverTimestamp - (serverTimestamp - clientTimestamp - nowTimeStamp + clientTimestamp - (nowTimeStamp - serverTimestamp))/2
serverTimeOffset = nowTimeStamp - serverTimestamp - (serverTimestamp - nowTimeStamp - nowTimeStamp + serverTimestamp)/2
serverTimeOffset = nowTimeStamp - serverTimestamp - (2serverTimestamp - 2nowTimeStamp)/2
serverTimeOffset = nowTimeStamp - serverTimestamp - (serverTimestamp - nowTimeStamp)
serverTimeOffset = nowTimeStamp - serverTimestamp - serverTimestamp + nowTimeStamp
serverTimeOffset = 2*nowTimeStamp

This maths are wrong but I don't know where:

serverClientRequestDiffTime = serverTimestamp - clientTimestamp;
serverClientResponseDiffTime = nowTimeStamp - serverTimestamp;
responseTime = (serverClientRequestDiffTime - nowTimeStamp + clientTimestamp - serverClientResponseDiffTime )/2;
serverTimeOffset = (serverClientResponseDiffTime - responseTime);
serverTimeOffset = nowTimeStamp - serverTimestamp - (serverClientRequestDiffTime - nowTimeStamp + clientTimestamp - serverClientResponseDiffTime )/2
serverTimeOffset = nowTimeStamp - serverTimestamp - (serverTimestamp - clientTimestamp - nowTimeStamp + clientTimestamp - (nowTimeStamp - serverTimestamp))/2
serverTimeOffset = nowTimeStamp - serverTimestamp - (serverTimestamp - nowTimeStamp - nowTimeStamp + serverTimestamp)/2
serverTimeOffset = nowTimeStamp - serverTimestamp - (2_serverTimestamp - 2_nowTimeStamp)/2
serverTimeOffset = nowTimeStamp - serverTimestamp - (serverTimestamp - nowTimeStamp)
serverTimeOffset = nowTimeStamp - serverTimestamp - serverTimestamp + nowTimeStamp
serverTimeOffset = 2*nowTimeStamp

try this:
A = B-C
D = E-B
G = (A-E+C-D)/2
F= D-G

F = (E-B)-(A-E+C-D)/2
= E-B -(B-C-E+C-E+B)/2
= E-B-(-2E+2B)/2 <== @ddcq
= E-B +E-B
= (E-B)*2