from datetime import datetime

 def timer_progress_tick(self, **event_args):
   """This method is called Every [interval] seconds. Does not trigger if
   [interval] is 0."""
     if self.start_time is not None:
       self.label_time_elapsed.text = "{}".format(
         datetime.strftime('%H:%M:%S', (datetime.now() - self.start_time))
       )