I did a quick test and it looks like the timer stops counting when the form becomes invisible and restarts from where it was when the form is displayed again.
I also found a little spooky bug: add a timer with the tick handler that prints something, run the app (the output panel shows the ticks), stop the app, delete the timer, run the app -> the output panel shows the ticks.
It is not a big problem because the first time the code is modified everything is back to normal.