You can already replay all your games on site with timestamps included. Oops! sorry, I see on chess.com you cannot see any timestamps of past gamaes (I've gotten used to other sites that include this). Unless, "analyze in Live" does this.
Wow! I'd not realize how hokey this site's games in history playback is. Well, that's not much fun so I think the site ought to include timestamps with on-site game histories.
Why would you need a pgn variation that could do the same off-site? Of what use would it be?
The one feature I feel is most lacking from the site is timestamped PGN. It would be awesome if the PGN files produced on the site include the time as displayed by the time clock on move completion for every move of the game.
I consider this an important one, and without actually seeing how the system is implemented, it should be relatively easy to add this to both Online and Live chess.
There seems to be some choice in implementation method, as noted here:
http://comments.gmane.org/gmane.games.board.chess.scid.user/5424