Run timer all the time

I think you can use a service module for this timer.

tried, not working good, since I need to display the timer on the original screen

You can use a callSub from the service module and update the label containing the timer in your activity. If of course I understand correctly. If you strictly measure time apart from doing something else with your timer, you can always hold a timerStart in a process_global variable, then get the current time, and report as elapsed time, the difference of these two variables.

Thanks,
I manage to creatr such code indifferent project, after I saw it is working fine, I integrated it into my code