Hi everyone,
I basically set this timer for 1 hour, and then, after opening other apps, I simply left the phone with the screen off. At a certain point, I went to pick it up from among the other running apps, and to my dismay, I realized that the timer was no longer running. 
TermosifoniTimer2.aia (899.5 KB)
You should really know by now that this would happen. Once the app moves to the background the clock will stop.
You can counter this by capturing the timer position when the clock stops, save this to tinydb. When you bring the app to the foreground get the current time and do maths to set the timer accordingly.
This extension can help you:
I know very well that the timer will stop in the background... but when I went to resume the app, the timer obviously started again. But after 1 hour, I saw that the timer had actually been disabled.
It is possible that your app closed, or that you timer ran out of time, and turned of the clock.