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);
    }
}

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