What is Lag Forgiveness? (Why did the clocks suddenly change?)

Don't worry - the clocks are not broken. This jumping is caused either by bonus time(increment) or because of Internet lag

In the case of internet lag, Chess.com (and most other chess sites) does something called 'lag forgiveness' where the time it takes for a move to travel to and from our servers is added back to the clocks.

Here is how lag forgiveness works on Chess.com:

When you make a move, the move has to spend some time traveling to our server, and then from our server to your opponent's computer. We adjust the clocks dynamically so that neither player is "charged" for the move's travel time - but rather, only for the time actually spent thinking.

Just to get a clear picture, let's imagine a very bad lag situation:

Imagine there is some very bad lag going on, and it takes 2 seconds for your move to get to our server, then another 2 seconds for the move to get to your opponent's computer from there. 

In this case, your opponent will not see this move until 4 seconds after you made it, but your opponent's clock will start counting down on your screen when the move hits our server. 

Then, let's say your opponent thinks for a total of 10 seconds, then plays a move. His move will take another 2 seconds to get to our servers, and another 2 seconds to go from our servers to your computer.

So, you have been waiting for this move for roughly 18 seconds (4 seconds travel time, 10 seconds thinking time, and another 4 seconds return travel time), and your opponent's clock on your screen has been counting down for roughly 16 seconds. 

Chess.com believes that it's unfair to charge someone for all that travel time! Since your opponent only spent 10 seconds thinking, that's all the time they get charged for. So, when their move finally hits your computer, their clock display adjusts for the 6 extra seconds that your opponent wasn't actually thinking. 

Meanwhile, your clock display on your end will only count down while you are thinking. There is no need for correction on your end, but on your opponent's screen they will be seeing the same adjustment to your clock, and possibly thinking there is something fishy going on there, if they haven't read this article!  

This lag or 'travel time' is what gets added back to your opponents clock AND your clock during games. Usually the travel time is very low, so you won't notice anything. But when someone has bad lag it can make it seem like the clocks are behaving badly! 

Fortunately, in reality, travel times are usually less than what is described above.  (In fact, we only allow members to play if their detected lag is two seconds or less.) However, the travel times are not consistent, so adjustments must be made dynamically. 

There is no way around it: all real-time chess servers have to deal with lag, though other sites are a bit less transparent about it!

Keywords: clocks, skip, clock, jumping, time, cheat, delay, bonus

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