Time sync between server and client

Question

Hi guys,
I have written a chess server and client. My next step is to implement a clock which will be used in timed games, however, I am not sure of the best solution. I can't think of a reliable method to keep the client's time in sync with the servers because the event latency varies between individual events and clients.
My first idea was to send a 'tick' event every second but I dont think this the best solution. I have seen many applet based clients which have timers, such as Yahoo chess.
I know this question is a little vague, I can give more detail if needed.
Thanks,
Alex

Answer 1

To prevent client-side cheating, the server should keep the
"official" clock. Every so often (perhaps when a move gets made?),
notify both clients of the official time. Clients count seconds from
that official time.
This is a suboptimal solution because it leaves clients vulnerable
to network lag. I can't think of any other solution that doesn't give
clients some responsibility for tracking time, though, and that's a
big security hole.
If you're willing to open that hole, have each client report back to
the server the elapsed local time from when they recieved the
last move to when they made their move. Then update the other
client's clock accordingly.