Synchronizing client-side and server-side times

Question

I have a webservices that collects realtime market data and displays it in a web frontend.

My frontend (jQuery) needs to display a clock, somewhat synchronized with the server clock (+- a few seconds is fine).

I was thinking of delivering a UTC timestamp alongside the rest of the data the server provides when the client loads.

Then, I would start a timer and every 1 seconds increment the displayed clock.

Is this a good approach or is it better to use a time server?

Thanks!

Looks good. You could also make additional requests to correct client side timer if there is no other requests for a while. — Pavel Reznikov, Jul 17 '12 at 11:08
If you don't care about a few seconds, what could be wrong with this approach? — Bergi, Jul 17 '12 at 11:33

score 12 · Accepted Answer · answered Jul 17 '12 at 11:24

12

You need to account for network latency. Data from your server won't reach you instantly. I suggest doing it using Cristian algorithm:

Client marks current client time (T1) and sends request to server.
Server sends it time (T2) to client.
Client receives server answer and marks its current time again (T3).
T3 - T1 is a total time request needed to go to the server and back (RTT). Now we can assume that going from server to client takes about half of that time. So, your "correct" time is T2 + (T3 - T1)/2

answered Jul 17 '12 at 11:24

Dmitry Osinovskiy

+1, although if +/- a few seconds is fine for him, he won't care about network latency. – Bergi Jul 17 '12 at 11:30
We don't know if it's fine for him. Anyway, it is all to easy to account for it - algorithm is very simple. – Dmitry Osinovskiy Jul 17 '12 at 11:35

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