code: def update_clock(self, dt):
          
          dt = dt.astimezone(self.tzinfo)
          fmt = "%H:%M"
          if self.show_seconds:
              fmt = "%H:%M:%S"
  
          self.time_txt.text = dt.strftime(fmt)
  
          suppl_text = "{0} {1}".format(dt.strftime("%Y-%m-%d"), self.timezone)
          self.suppl_txt.text = suppl_text
  
          self.viewer.redraw(whence=3)
docs: This method is called by the ClockApp whenever the timer fires
          to update the clock.  `dt` is a timezone-aware datetime object.