As all modern OS implement some kind of multi tasking, when you call sleep, your process will be set to sleeping state until the sleep time has elapsed. But it will not reactivate your process exactly after 1 second, instead it will be 1 second + some more or less random x.
So this timer is not accurate especially for long durations.
The better way to implement this would be to save your current system time when you click start (e.g. with starttime = datetime.datetime.now()). Then while running, substract current time from starttime: