JS for simulating NTP time-sync. Original from: https://jehiah.cz/download/NTP.js, but didn't work - so this one has several edits

js-ntp.js

/*
NTP.js https://jehiah.cz/a/ntp-for-javascript
copyright Jehiah Czebotar jehiah@gmail.com
licensed under http://unlicense.org/ please modify as needed
to use configure serverUrl to an endpoint that when queried
(with X=timestamp_in_miliseconds)

GET serverUrl + '/' + X

returns a plain-text body:

(server_time_in_milliseconds - X) + ':' + X

*/
var NTP = {
    cookieShelfLife: 3600, // 1 hour
    requiredResponses: 10,
    serverTimes: new Array,
    serverUrl: "https://your-server/",
    resyncTime: 10, // minutes
    sync: function () {
        // commented for testing
        // if the time was set within the last x minutes; ignore this set request; time was synce recently enough
        // var offset = NTP.getCookie("NTPClockOffset");
        // if (offset){try{
        //     var t = offset.split("|")[1];
        //     var d = NTP.fixTime()-parseInt(t,10);
        //     if (d < (1000 * 60 * NTP.resyncTime)){return false;} // x minutes; return==skip
        // }catch(e){}
        // }
        NTP.serverTimes = new Array;
        NTP.getServerTime();
    },
    getNow: function () {
        return Date.now();
    },
    parseServerResponse: function (data) {
        var offset = parseInt(data.responseText.split(":")[0]);
        var origtime = parseInt(data.responseText.split(":")[1]);
        var delay = ((NTP.getNow() - origtime) / 2);
        offset = offset - delay;
        NTP.serverTimes.push(offset);

        // if we have enough responces set cookie
        if (NTP.serverTimes.length >= NTP.requiredResponses) {
            // build average
            var average = 0;
            var i = 0;
            for (i = 0; i < NTP.serverTimes.length; i++) {
                average += NTP.serverTimes[i];
            }
            average = Math.round(average / i);

            NTP.setCookie("NTPClockOffset", average); // set the new offset
            console.log('Track fixTime', NTP.fixTime(), new Date(NTP.fixTime()).toString())
            console.log('Track not fixTime', Date.now(), new Date(Date.now()).toString())
        } else {
            NTP.getServerTime();
        }
    },
    getServerTime: function () {
        try {
            var xhr = new XMLHttpRequest();

            // Making our connection
            var url = NTP.serverUrl + Date.now();
            xhr.open("GET", url, true);
            // function execute after request is successful
            xhr.onreadystatechange = function (res) {
                if (this.readyState == 4 && this.status == 200) {
                    NTP.parseServerResponse({responseText: this.responseText});
                }
            }
            // Sending our request
            xhr.send();
        } catch (e) {
            return false;
            //prototype.js not available
        }
    },
    setCookie: function (aCookieName, aCookieValue) {
        var date = new Date();
        date.setTime(date.getTime() + (NTP.cookieShelfLife * 60 * 1000));
        var expires = '; expires=' + date.toGMTString();
        document.cookie = aCookieName + '=' + aCookieValue + expires + '; path=/';
    },
    getCookie: function (aCookieName) {
        var crumbs = document.cookie.split('; ');
        for (var i = 0; i < crumbs.length; i++) {
            var crumb = crumbs[i].split('=');
            if (crumb[0] == aCookieName && crumb[1] != null) {
                return crumb[1];
            }
        }
        return false;
    },
    fixTime: function (timeStamp) {
        if (!timeStamp) {
            timeStamp = NTP.getNow();
        }
        var offset = NTP.getCookie("NTPClockOffset");
        try {
            if (!offset) {
                offset = 0;
            }
            temp = parseInt(offset, 10);
            if (isNaN(temp)) {
                return timeStamp;
            }
            return timeStamp + temp;
        } catch (e) {
            return timeStamp;
        }
    }
}
console.log('Track Before NTP.sync', NTP.fixTime(), new Date(NTP.fixTime()).toString())
setTimeout('NTP.sync()', 2500);