Netlag often causes players to lose valuable seconds on their chess clocks. Transmission time is counted against you, unless the chess server can tell exactly when information is transmitted. What timeseal does is record your thinking time, so that transmission time is not counted against you. Timeseal will not prevent netlag but it makes the games fairer when lag does occur.
I've been informed chess.com does not have timeseal, or timestamp, or anything equivalent. If this is incorrect someone please let me know.
Timeseal
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As described here, the client - here your webbrowser - sends the move you made and the time you thought about it. the problem may be that this time value is easy hackable, before the browser sends it (because javascript is not compiled - it is also hackable in other software but its not as trivial). That may be the reason why chess.com hasnt done this yet. But they also may just have not the skills or the people to do it currently.
Hey! I forgot all about that page!
I haven't seen that page of mine in YEARS! As I recall, I put it together when I was administrator on that server.
[COMMENT DELETED]
February 28, 2003! That's the date of that file, on my hard drive! That article is a bit more than ten years old! And it's probably been seven years or so, (or more) since I've seen it.
Yea, thanks to technology, the world today is indeed a small place.
the link remarks "If you are continuously having a problem with lag, you may wish to consider changing your Internet provider. A different provider will have a different route to USCL which will probably be less troublesome."
A far simpler approach is if you're unhappy with your lag you can try reconnecting to chess.com, it's unlikely the same route will be chosen and perhaps your lag will be better (or worse) next time around.
sftac
Ah, yes. But ten years ago most everyone didn't have high speed connections. Back then we were using dial-up connections with our phone lines, often with, if you were lucky, 56 k modems. Back then local internet providers were a dime a dozen. Some of these companies were good... with good service, while others were just in it to try to make a quick buck. Constantly hanging up your phone and redialing to reconnect wouldn't have been ideal. 
What is the timeseal?
Do chess.com has a timeseal?