No youngster in my parents houseYou misunderstand me. I meant someone purposely changing the time as a prank. If there are any youngsters around, I would consider this one of the most likely possibilities.
Bob
SMPS is there before the lineart (to convert AC/DC). Now I am playing with debounce time. Debounce limit of longer than 90 ms seems to work (For 4 days now).I have no idea if you ever fixed it, but I would put a switching power supply in there.
To avoid problems with kids, I'd create some sort of "trick" that's necessary to set the time. Magnet. Some "code" using the existing buttons that might enable setting for a certain amount of time.
make sure bypass caps are used.