The best way to synchronize client-side javascript clock with server date

You can calculate exact time with NTP (Network Time Protocol) in your codes,

i try to explain for you:

1. We have ClientTime on Sending Request (for example 4/3/2012 13:56:10.123)
2. You send ClientTime to Server
3. We have Round-trip time for request, i called it RequestTime (for example: It takes 5 seconds)
4. In Server, We calculate Difference time between Server and Client (for example: It ServerTime – ClientTime = ServerClientDifferenceTimeWithRequestTime), you should now this Difference including Round-trip request time in step 3 then you should remove round trip time from Difference
5. Server Send response that include ServerClientDifferenceTimeWithRequestTime and ServerTime
6. We have Round-trip time for response, i called it ResponseTime (for example: It takes 3 seconds)
7. In client, We calculate Difference time between Server and Client again (for example: It ServerTime – ClientTime = ServerClientDifferenceTimeWithResponseTime), again: you should now this Difference including Round-trip response time in step 6
8. We have Now time in Client
9. You should Calculate simple equations in client:

X(SyncedTime) = Now + (ServerClientDifferenceTimeWithRequestTime - RquestTime)

X(SyncedTime) = Now + (ServerClientDifferenceTimeWithResponseTime - ResponseTime)

Now - ClientTime = RquestTime + ResponseTime =>

Now - (ServerClientDiffRq - RquestTime) = Now - (ServerClientDiffRs - ResponseTime)

if you solve it you found this:

ResponseTime = (ServerClientDifferenceTimeWithRequestTime - Now + ClientTime + - ServerClientDifferenceTimeWithResponseTime )/2

and then you can found synced time or server time in client with this equation:

X(SyncedTime) = Now + (ServerClientDifferenceTimeWithResponseTime - ResponseTime)

I show simple code but when you want write it don`t forgot use UTC date & time functions…

Server Side (for example php, c#):

PHP:

header('Content-Type: application/json; charset=utf-8');
$clientTime = $_GET["ct"] * 1; //for php 5.2.1 or up: (float)$_GET["ct"];
$serverTimestamp = round(microtime(true)*1000); // (new DateTime())->getTimestamp();
$serverClientRequestDiffTime = $serverTimestamp - $clientTime;
echo "{\"diff\":$serverClientRequestDiffTime,\"serverTimestamp\":$serverTimestamp}";

C#:

long clientTime = long.Parse(Request.Form["ct"]);
long serverTimestamp = (DateTime.Now.Ticks-(new DateTime(1970,1,1) - DateTime.MinValue).Ticks) / 10000;
long serverClientRequestDiffTime = serverTimestamp - clientTime;
Response.Write("{\"diff\":"+serverClientRequestDiffTime+",\"serverTimestamp\":"+serverTimestamp+"}");

Client Side (Javascript with Jquery):

var clientTimestamp = (new Date()).valueOf();
$.getJSON('http://yourhost.com/getdatetimejson/?ct="+clientTimestamp, function( data ) {
    var nowTimeStamp = (new Date()).valueOf();
    var serverClientRequestDiffTime = data.diff;
    var serverTimestamp = data.serverTimestamp;
    var serverClientResponseDiffTime = nowTimeStamp - serverTimestamp;
    var responseTime = (serverClientRequestDiffTime - nowTimeStamp + clientTimestamp - serverClientResponseDiffTime )/2

    var syncedServerTime = new Date((new Date()).valueOf() + (serverClientResponseDiffTime - responseTime));
    alert(syncedServerTime);
});