A potential new player recently asked the club I’m associated with:
“Are chess clock apps considered OK during a tournament? Especially if a tablet is used so the clock and buttons are large.”
What should I tell him?
In short, no.
Maybe there are directors and opponents that would let it slide, but it is not something that is standard equipment. We also don’t allow electronic devices, including the tablet used to run the application.
I would force players to play without a clock before allowing a chess clock app.
No–see rule 20N. But if an opponent does not object, then I allow it. The danger is the device’s ability to have either (1) a chess playing program installed on the device or (2) the device can contact an outside source which uses a chess engine. The suspicion of cheating with such devices far outweighs the times it really happens. But when it does happen everyone involved gets a headache.
The only time I’ve ever allowed it was at a non-rated tournament where I had forgotten to bring a clock for long games. I used my phone so that I knew no chess engine was running.
If there is no clock available, and both players agree to the use of the app, why not? I have seen a couple of these apps and they work pretty well, and are easier to read than some digital clocks. I do warn the players that when time gets low that the phone with the app is going to take a beating and may spin off the table to the floor. The chances that the app is connected to confederates who have access to a computer program and powerful computer is minimal. Both players are observing the device. If someone is cheating, he would do it in a more devious way. The times I have seen the use of a clock app, the phone was pretty sturdy, the players were young techie guys new to the game. Didn’t see any reason to discourage their play. The guys using an old BHB clock were more of a problem when time pressure hit.
The biggest problem I see is that one player may feel pressured to agree to use a clock app (versus not having a clock) and after losing the game will claim that the diabolical, lying, cheating opponent used that pressure to get a chess engine accepted as a clock. and the player may well fully believe the accusation he is making. On average, a parent or coach making the same accusation is even more likely to fully believe the accusation.
A firm line on using such clock apps will eliminate those high-tension accusations. If you are going to allow them then it is safest (from the accusation standpoint, not from the equipment damage standpoint) to provide the devices for those clock apps at the start of the game. Simplest is to simply not allow them.
In a casual club environment a clock app may be feasible (assuming, of course, that your club has never had issues with players ever thinking an opponent did something underhanded).
Anyone dreaming of doing something underhanded at a tournament should call up the TD to apologize. 
No.
I am sure some parents and kids would phrase it that way, but just
because you do not have an approved clock to lend does not mean that you are forcing them to play without one. Actually most tournaments I know of do not supply clocks.
Rob Jones