JSFiddle - React, Tailwind, and code Playground

HTML

Serverzeit: <span class="clock"></span>

JavaScript

var server_time = 1506360488; // Hier server timestamp einsetzen
var client_time = Date.now()/1000;

function zeropad(s,n){
  s = s.toString();
  while( s.length < n )
    s = '0'+s;
  return s;
}

function buildDateString(d){
  var time = zeropad(d.getUTCHours(),2)
     + ':' + zeropad(d.getUTCMinutes(),2)
     + ':' + zeropad(d.getUTCSeconds(),2);
  var date = d.getUTCFullYear()
     + '-' + zeropad(d.getUTCMonth()+1,2)
     + '-' + zeropad(d.getUTCDate(),2);
  return date + ' ' + time;
}

for( var clock of Array.from(document.getElementsByClassName("clock")) ){
  var updateTime = function(){
    var d = new Date((server_time-client_time)*1000+Date.now());
    clock.innerHTML = buildDateString(d);
  };
  setInterval(updateTime,1000);
  updateTime();
}