First, let's clear up the matter that a computer operating system knows what a second is and that any second or fraction of a second must be the same measure of time, in fact.  

Keep in mind on the server, the system clock and the game clock are different, one, the former mentioned, can be manipulated and one, the later, can not, at least, without affecting everything running.  

Considering that the game clock can be manipulated*, by consent, i.e., 3/1 (increamental chess) and premove (1/10 of a second per move) and that the packettes that transfer your game information back and forth to the server are UDP (non secure), sure it is possible to cheat, by clock manipulation.  In fact if you look up Winboard, and understand code, you can see that there are already some nice working time handicapping routines written in to it.

Can chess.com do some thing about it?  Not very easily.

*Think of the game clock as a digital egg timer, that can be manipulated, just as your egg timer, but the manipulation is via software instruction.  It can be hacked.

well in my opinion lag is lag,2 expound on what El senior said concerning Tsmith's comment about clock cheating,what has really become undermined is the fact that internet speed becomes important in close games,even if u have the same operating system,one must remember that everyone's computer doesn't posess the same internet speed,and these games are being played via internet,therefore in close games whoever has the system with the fastest speed,i believe the clock responds accordingly and tenths of a second or even seconds begin 2 matter in who wins,aside from the fact that there is something particularly strange about the ticking of the clock on occasion.