In the old days of mechanical chess clocks there were "buttons" with stems which were pushed to start your opponents clock. These buttons could be seen for some distance allowing one to walk about while keeping watch on their clock out of the corner of their eye.The Excaliber clock is an example of that feature being carried over to the new digital timers.Recently I have noticed several players at tournaments who turn off the move indicator lights on their chess clocks. Indeed I was even thanked by an elderly opponent in last years Louisville Open for having them turned on on my Chronos. I personally consider turning the lights off unsporting; therefore I decided to see if there were any rules regarding this. While USCF does not cover this in their rules, FIDE does. Reference FIDE Handbook 5.1.(d) covers clocks. Which states that you should be able to see who is on the move from a distance of 10 meters.But more specific is the testing requirements to get FIDE approval of a new clock:"Clocks must provide a clear visibility of which player is to move, from all sides at a distance of 10 meters".
As USCF gradually gets more instep with FIDE, I predict that "clock rules" will be upgraded to conform with FIDE standards. Something you should consider if in the market for a clock. Currently the DGT XL Digital Chess Clock is the only FIDE approved clock sold by USCF.
There is no need to panic and trash your old clock; any clock can be used in a non-FIDE sanctioned event. But if you use a Chronos, be a good sport and turn on the LED move indicators!