I'm a chess player and programmer. There was actually a paper on this 10 years ago about the timestamp and other security on ICC. You should read it with the understanding that things may be different now:
How to Cheat at Chess:
A Security Analysis of the Internet Chess Club
But in general, you are correct. It is possible to manipulate the times reported in timeseal (timestamp). I've actually done it, even with ICC's official interface, just to see if it can be done. Speed hacks exist for many games and I used an existing speed hack program to slow down my clock instead of speeding it up (which you normally would do to run faster in a game). The result was a game I played vs KBNK (a bot which lets you practice B+N mate), in which I mated the computer after about 30 moves in less than 3 seconds played time. Averaging 0.1 seconds per move or so on a non-trivial mate is definitely unnatural.
I reported the effect to the administrators of the site, but I was told they were able to detect it. In fact, they had a log of my fraudulent time stamps detected (somewhere... I assume they can see blatant false reports, as well as compare it to TCP/IP timestamps and see if the numbers are reasonable). I was told that they were able to detect such cheating and to please not use it but thanks for my concern.
So, yes it is possible to manipulate anything client side, but it is also possible that they can detect fraudulent time stamps server side. Better chess servers typically already have a team of people that investigate reports of cheating, either by computer or clock manipulation.