I have been playing chess with my son quite a bit lately. It happens from time to time that one of us will take quite some time between moves so we thought it would be nice to play with a clock. Which of course we don´t have and it makes no sense to buy one.
However it is easy to create a simple one so I just made an app exactly for this. I know, there are already many apps doing the same. However I know this one is not tracking me, not trying to sell me anything and not selling my data to third parties.
If you want to give it a try (Android), the app is available in the MIT´s App Inventor gallery
PS: Updated so that each player has different starting times and also to make it more obvious whose time is still running (button shown as red)
PPS (10th Feb): Updated with Pause and Continue buttons as some times we are interrupted while playing. While paused, Light and Dark buttons show in orange and have no effect until you press the Continue button.