Freechess shipped a binary called “timeseal” that did the calculations for you and encrypted the communications. It was not foolproof—-not by a long shot, but it also didn’t completely suck.

You can read about what became timeseal here. https://eprint.iacr.org/2004/203.pdf

Well it's a long time since I played there. But it had custom chess clients, which I assume just recorded how much time your move actually took and sent that with the move.

Yes, it's easy to cheat with this, but it's very easy to cheat with chess anyway.