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    <p>I recently took a one-hour class on this and it wasn't long enough, but I'll try to boil it down to get you pointed in the right direction. Get ready for a little algebra.</p> <p>Let s equal the time according to the server. Let c equal the time according to the client. Let d = s - c. d is what is added to the client's time to correct it to the server's time, and is what we need to solve for.</p> <p>First we send a packet from the server to the client with a timestamp. When that packet is received at the client, it stores the difference between the given timestamp and its own clock as t1.</p> <p>The client then sends a packet to the server with its own timestamp. The server sends the difference between the timestamp and its own clock back to the client as t2.</p> <p>Note that t1 and t2 both include the "travel time" t of the packet plus the time difference between the two clocks d. Assuming for the moment that the travel time is the same in both directions, we now have two equations in two unknowns, which can be solved:</p> <pre><code>t1 = t - d t2 = t + d t1 + d = t2 - d d = (t2 - t1)/2 </code></pre> <p>The trick comes because the travel time is not always constant, as evidenced by your pings between 50 and 200 ms. It turns out to be most accurate to use the timestamps with the minimum ping time. That's because your ping time is the sum of the "bare metal" delay plus any delays spent waiting in router queues. Every once in a while, a lucky packet gets through without any queuing delays, so you use that minimum time as the most repeatable time. </p> <p>Also keep in mind that clocks run at different rates. For example, I can reset my computer at home to the millisecond and a day later it will be 8 seconds slow. That means you have to continually readjust d. You can use the slope of various values of d computed over time to calculate your drift and compensate for it in between measurements, but that's beyond the scope of an answer here.</p> <p>Hope that helps point you in the right direction.</p>
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    1. USKarl Bielefeldt
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    1. COWow! Thanks! That's exactly what I'm looking for. It's not entirely clear to me yet why that's better than my original method (although I believe that it is), but I'll post back here when I try it out. I don't think I'll need to adjust d throughout a gaming session, but it shouldn't be hard to do if I need to. Thanks again!
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    2. COThis has worked really well. I found that if I always take the smallest (i.e., most negative) t1 and t2 values, it's the same as taking the smallest pings, so it converges on the correct offset. I have t1 = client stamp - server stamp and t2 = server stamp - client stamp Also, I've found out that iphones drift pretty significantly! Within a half an hour, the actual offset between two devices might change by as much as half a second. I verified this with clock programs that contact NTP servers.
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    3. COGlad it was helpful for you.
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