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

server_time_sync.js

// Original from http://stackoverflow.com/questions/1638337/the-best-way-to-synchronize-client-side-javascript-clock-with-server-date
// Improved by community and @jskowron

// Synchronize client-side clock with server time using an approximation of NTP
let serverTimeOffset = null;

function getServerTime(callback) {
  if (serverTimeOffset === null) {
    const scripts = document.getElementsByTagName("script");
    const currentScriptURL = scripts[scripts.length - 1].src;

    const clientTimestamp = Date.now(); // Client timestamp before request
    const xhr = new XMLHttpRequest();

    xhr.open("HEAD", `${currentScriptURL}?noCache=${clientTimestamp}`, true);

    xhr.onload = function () {
      if (xhr.readyState === 4 && xhr.status === 200) {
        const serverDateHeader = xhr.getResponseHeader("Date");

        if (serverDateHeader) {
          const serverTimestamp = Date.parse(serverDateHeader);
          const nowTimeStamp = Date.now(); // Timestamp after response

          // Calculate offset
          serverTimeOffset = serverTimestamp - ((clientTimestamp + nowTimeStamp) / 2);

          // Return synchronized time
          const syncedDate = new Date(Date.now() + serverTimeOffset);
          if (callback) callback(syncedDate);
        } else {
          console.error("Server did not return a valid Date header.");
        }
      } else {
        console.error("Failed to fetch server time:", xhr.statusText);
      }
    };

    xhr.onerror = function () {
      console.error("Network error while fetching server time.");
    };

    xhr.send(null);
  } else {
    // Return cached offset-adjusted time
    const syncedDate = new Date(Date.now() + serverTimeOffset);
    if (callback) callback(syncedDate);
  }
}

@jskowron Thank you for be patient with me. Actually at that time I'm very busy until point that my brain isn't working properly.
Today I read your message carefully again and I agreed with you so I improved code.

Hope that next year will be happy years for everyone and thank you for who helped to contribute in this little code snippet.