def update_clock(self, dt):
        """This method is called by the ClockApp whenever the timer fires
        to update the clock.  `dt` is a timezone-aware datetime object.
        """
        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)
This method is called by the ClockApp whenever the timer fires
        to update the clock.  `dt` is a timezone-aware datetime object.